There Are More Stars Than Grains Of Sand _ But The Universe Is Almost Completely Empty

Welcome to the Science for Sleep channel, where we explore the wonders of the universe in a calm and relaxing way. Before we begin, the atoms in your body right now are almost entirely empty space. The matter in you, compressed to its actual density, would fit inside a grain of salt. The universe containing you is 100% nothing. And yet here you are, taking up space, aware of the emptiness. Tell us where you're watching from, your city, your country, what time it is, whether the room is dark and quiet or the world outside is still going. Somewhere in a universe that is almost completely empty you are watching this.

Drop it in the comments, we read every one. Pick up a handful of sand from a beach. Let it run through your fingers. Count the grains not literally, but imagine counting them. A single handful of dry sand contains somewhere between 5,000 and 10,000 individual grains, depending on the coarseness of the sand and the size of your hand.

A small bucket, the kind a child brings to the beach, contains perhaps a million. Now scale up. A cubic metre of sand contains approximately 8 billion grains. A stretch of beach one kilometre long, 50 metres wide and 2 metres deep contains roughly 800 trillion grains. All the beaches on Earth, every coastline, every desert shoreline, every sandy bank of every river are estimated to contain approximately 7.5 times 10 to the power of 18 grains of sand. That is 7.5 quintillion.

A 7 followed by 18 zeros. This number is used so often as a benchmark for incomprehensible quantity that it has almost lost its ability to impress. 7.5 quintillion grains of sand on Earth. We have heard this before. We nod and move on. But here is the number that should stop you. The observable universe contains an estimated 200 billion trillion stars written out, that is 2 times 10 to the power of 23, a 2 followed by 23 zeros.

This estimate, refined over decades of astronomical observation and published in a landmark 2016 study in the Astrophysical Journal, it is considered conservative. It accounts only for stars in galaxies we can detect, using detection methods that likely miss significant numbers of faint, small or distant stars. Compare the two numbers directly. 7.5 quintillion grains of sand on Earth. That is 7.5 times 10 to the power of 18. 200 billion trillion stars in the observable universe. that is 2 times 10 to the power of 23. The number of stars exceeds the number of grains of sand on Earth by a factor of approximately 10,000. For every single grain of sand on every beach and desert on Earth, there are roughly 10,000 stars in the observable universe. This is

the comparison that tends to produce a particular kind of silence in people who sit with it properly. Not the silence of confusion, but the silence of something genuinely landing. 10,000 stars per grain of sand. Walk along a beach and pick up a single grain. Turn it in your fingers. That grain represents, in the stars of the observable universe, not one star, not a handful, but 10,000. A private galaxy's worth for each grain. but the comparison extends further than most people take it. The observable universe is not the universe. It is the portion of the universe from which light has had time to reach us since the Big Bang, a sphere of visibility centred on Earth, defined by the speed of light and the age of the cosmos.

Beyond it, the universe continues, in all likelihood, vastly further than the observable portions. If the universe is even moderately larger than the observable portion, the total number of stars it contains dwarfs the observable count by factors that themselves have no meaningful comparison. And stars are not the only objects to count. Many stars have planets. Current estimates suggest the Milky Way alone contains more planets than stars, perhaps several hundred billion planets in our galaxy. The observable universe's 200 billion trillion stars at similar ratios imply something in the range of several hundred billion trillion planets. Moons orbiting those planets add another order of magnitude. Asteroids, comets and

smaller bodies multiply the count further. The observable universe contains more individual rocky bodies than there are grains of sand on Earth. by a factor so large it has no intuitive analogy. And yet, if you took every single one of those stars, all 200 billion trillion of them, and placed them together, the total volume they occupy would be a vanishingly small fraction of the observable universe's total volume.

The stars, for all their number, are islands of matter in an ocean of nothing. The universe is not full of stars, The universe contains stars the way the ocean contains fish present, real, occasionally numerous in local clusters, but utterly overwhelmed by the medium they inhabit. The number is real.

The fullness it implies is not. The universe has more stars than Earth has grains of sand, and it is almost completely empty. Before we measure the emptiness of space, we need to confront something more personal. The matter that fills space, the stars, the planets, the gas clouds, everything we point to when we say the universe contains something is itself almost entirely empty. Start with the atom. Every piece of matter you have ever touched, every object you have ever seen, every planet and star and galaxy in the observable universe is made of atoms. Atoms are the fundamental units of ordinary matter, and atoms are, by volume, almost completely nothing. A hydrogen atom, the simplest and most

abundant atom in the universe, consists of a single proton at its center and a single electron in a cloud around it. The proton has a radius of approximately 0.85 femtometers. The electron cloud extends to a radius of approximately 53 picometers, the Bohr radius. A picometer is 1,000 femtometers. The electron orbits at a distance roughly 60,000 times the radius of the proton it orbits. Scale this up to something visible.

If the proton at the center of a hydrogen atom were the size of a marble roughly one centimeter across the electron would be orbiting at a distance of approximately 600 meters away, half a kilometer. The marble sits at the center of a sphere half a kilometer in radius, and somewhere on the surface of that sphere, a particle far smaller than the marble traces its path through otherwise empty space. Everything between the marble and the distant electron is nothing, no matter, no substance. just the quantum fields that govern their interaction real in a physical sense, but not matter in any conventional meaning of the word. This is not a peculiarity of hydrogen. Every atom, in every element, has the same basic architecture, a tiny dense nucleus surrounded

by an electron cloud that defines the atom's size but contains almost none of its mass. separated from the nucleus by a vast proportional emptiness. The nucleus of any atom contains more than 99.9% of the atom's mass, in less than one trillionth of its volume. The rest, the overwhelming majority of the space the atom occupies, is effectively empty.

Your body is made of atoms. Every atom in your body is structured this way, If all the empty space inside every atom in your body were removed, if all the nuclei and electrons were compressed together with no space between them, the remaining matter would fit inside a single grain of salt. Every human being who has ever lived compressed to nuclear density would fit inside a sugar cube.

The matter you are made of is almost entirely empty space. And that matter is itself embedded in a universe that is almost entirely empty space at every larger scale. The emptiness goes all the way down. The solar system feels full. Eight planets, dozens of moons, hundreds of thousands of asteroids, billions of comets, The vast Oort cloud surrounding everything a rich, complex, populated system built around a central star. It is not full.

It is almost incomprehensibly empty. Start with the Sunday. The Sun contains 99.8% of all the mass in the solar system. Everything else, every planet, every moon, every asteroid, every comet, every dust grain accounts for the remaining 0.2%. Jupiter alone contains two-thirds of that 0.2%. The remaining seven planets, all their moons, the entire asteroid belt, the Kuiper belt, and the Oort cloud, share the last fraction of that last fraction.

But mass is not volume. The question of emptiness is about volume. about how much of the solar system's total space is actually occupied by something rather than nothing. The observable solar system, including the Oort cloud at its outer boundary, occupies a sphere approximately two light years in diameter. That is a sphere of roughly 10 to the power of 38 cubic meters of space. The total volume of all the objects in the solar system, every planet, Every moon, every asteroid, every known body adds up to approximately 10 to the power of 27 cubic meters. The sun alone accounts for the vast majority of that volume. Divide the volume of matter by the volume of the solar system.

The result is approximately one part in 10 to the power of 11. One part in 100 billion. The solar system is roughly 99.999999999% empty space. The matter it contains occupies less than one billionth of one percent of its total volume. To visualize this differently, if you shrunk the solar system to the size of Earth, all the matter in it would be invisible to the naked eye. Every planet, every moon, every asteroid reduced to subatomic scale by the same reduction that brings the solar system down to Earth size.

The space between objects is not incidental to the solar system. It is the solar system. The objects are the incidental part. The asteroid belt is perhaps the most instructive example. In science fiction, Asteroid belts are depicted as dense fields of tumbling rocks that spacecraft must carefully navigate a gauntlet of collisions and close calls. The reality is almost the opposite.

The asteroid belt contains hundreds of thousands of asteroids, but they are distributed across a region of space so vast that the average distance between any two asteroids is roughly 600,000 kilometres. Several spacecraft have passed directly through the asteroid belt without any special navigation precautions. The probability of randomly encountering an asteroid while passing through is essentially zero. The solar system is not a crowded place. It is an almost perfectly empty place, with small islands of matter separated by distances so large that travelling between them takes years. even at the speeds of our fastest spacecraft. And the solar system is one of the denser regions of the galaxy. The

solar system is 99.999999999% empty space. That already dismantles any intuition that matter is the dominant feature of the cosmos. But the solar system, for all its emptiness, is a region of extraordinary local density compared to the galaxy it sits inside. The Milky Way contains between 200 and 400 billion stars. It is approximately 100,000 light years across and roughly 1,000 light years thick at the position of our solar system. Its total volume is enormous, approximately 8 times 10 to the power of 60 cubic metres. Now consider the stars inside it. The Sun has a radius of approximately 696,000 kilometers large by human standards, negligible by galactic ones.

If every star in the Milky Way were the size of the Sun, the total volume occupied by all 400 billion stars would be approximately 10 to the power of 38 cubic meters. Divide that by the galaxy's total volume. The stars of the Milky Way occupy approximately one part in 10 to the power of 22 of the galaxy's total volume. That is one part in 10 billion trillion.

The Milky Way is so empty that if you compressed all its stars together, every single one of the 400 billion, the resulting object would be smaller than a grain of sand, compared to the galaxy's total size. The average distance between stars in the Milky Way is approximately four to five light years, roughly the distance to Proxima Centauri, our nearest neighbor.

At our scale from the grain of sand video, where the Sun is a grain of sand, that distance is 30 kilometers. Two grains of sand, 30 kilometers apart, in a desert that extends 70 million kilometres in every direction. That is the density of the Milky Way. But stars are not the only matter in the galaxy. The interstellar medium, the gas and dust filling the space between stars, adds mass without adding much volume. On average, the interstellar medium contains approximately one hydrogen atom per cubic centimetre. One atom per cubic centimetre. The best vacuum ever achieved in a laboratory on Earth, 10 to the power of minus 17 atmospheres, contains far more particles per cubic centimeter than the average

interstellar space. The space between stars is not merely empty in the everyday sense of the word, it is empty in a way that has no analogy in human experience. No chamber, no container, no environment ever encountered by any human being has come close to the emptiness of interstellar space. The air in this room contains approximately 10 to the power of 19 molecules per cubic centimetre. Interstellar space contains one atom per cubic centimetre. The Milky Way is a structure defined almost entirely by what it doesn't contain. Its stars are punctuation marks in a sentence made almost entirely of silence. The Milky Way is almost entirely empty. Its 400 billion stars occupy a fraction of its volume, so

small it has no everyday analogy. And the Milky Way is one of two trillion galaxies in the observable universe. Now zoom out to the largest scale we can observe. The observable universe has a diameter of approximately 93 billion light-years. Its total volume is roughly 4 times 10 to the power of 80 cubic metres. Inside that volume sits all the matter we can detect. Every galaxy, every star, every planet, every atom of gas and dust, in every interstellar medium of every galaxy that has ever existed within our observable horizons.

The total amount of ordinary matter in the observable universe, the matter made of protons, neutrons, and electrons that constitutes everything we can see and touch, is estimated at approximately 10 to the power of 53 kilograms, vast beyond any human scale. But matter is dense. Convert that mass to volume at average stellar density. and all the ordinary matter in the observable universe occupies approximately 10 to the power of 47 cubic meters. Divide by the universe's total volume.

Ordinary matter occupies approximately one part in 10 to the power of 33 of the observable universe. One part in a billion trillion trillion. If you took all the matter in the observable universe, every star, every planet... every gas cloud, every atom of every element ever forged in every stellar furnace across 13.8 billion years of cosmic history and spread it uniformly through the universe's volume, the average density would be approximately six hydrogen atoms per cubic metre. Not per cubic centimetre, per cubic metre. Six atoms in a space the size of a large refrigerator. And this is the average.

including the relatively dense regions of galaxy clusters and filaments. The universe's large-scale structure is not uniform. It is organized into a cosmic web, a foam-like architecture of filaments and sheets of galaxies surrounding enormous empty regions called voids. The filaments, where galaxy clusters concentrate, are the densest regions. but they are separated by Void's vast spherical regions of space containing almost no galaxies at all. The Boötes Void is one of the largest known, a roughly spherical region approximately 330 million light years across, containing almost no galaxies. To put that in context, the entire observable universe is 93 billion light years across. The boat's void is 330 million light

years of almost nothing. A bubble of emptiness so large that if the Milky Way were at its centre, we would not have known other galaxies existed until the 1980s. Because the nearest ones would have been at the void's edge. These voids are not rare. They constitute the majority of the universe's volume. The filaments and walls where galaxies cluster are the exceptions. The voids are the rule. The universe is not a place filled with galaxies. It is a place filled with nothing, through which thin threads of galaxies are strung like cobwebs across an infinite dark room. Everything we have covered so far points in one direction. Atoms are mostly empty. The solar system is mostly empty. The galaxy is mostly empty. The universe is mostly empty. The

voids between galaxy filaments are emptier still. Emptiness, at every scale, is the dominant feature of physical reality. And yet the void is not empty. This is where the story takes its strangest turn. Because the space between things, the space we have been calling empty, the space that constitutes the overwhelming majority of the universe is not nothing. It is something.

Something active, something dynamic, something that physicists are still struggling to fully understand. Start with dark matter. Approximately 27% of the universe's total energy content is dark matter. A substance that does not emit, absorb or reflect light. Interacts with ordinary matter only through gravity. And has never been directly detected despite decades of searching. Dark matter is not in the stars and galaxies. It is distributed throughout the universe in enormous halos surrounding galaxies and filling the spaces between them.

The voids that appear empty to our telescopes are threaded with dark matter invisible, undetectable by any current instrument, but gravitationally present, shaping the large-scale structure of the cosmos. Then there is dark energy. Approximately 68% of the universe's total energy content is dark energy, the mysterious force driving the accelerating expansion of the universe. Dark energy is not concentrated in objects or structures. It is a property of space itself present everywhere, in every cubic meter of the universe, including the most empty voids, pushing space apart at an accelerating rate. The emptier the region, the more dark energy it contains in absolute terms, because dark energy scales with

volume rather than mass. The void between galaxies is, by dark energy content, the richest environment in the universe. Then there is quantum vacuum energy. According to quantum field theory, empty space is not truly empty at the quantum level. The vacuum is filled with quantum fields, the fundamental entities that give rise to all particles and forces, and these fields are never truly at rest. They fluctuate constantly, producing pairs of virtual particles that pop into existence and annihilate almost instantly. Too brief to detect directly, but real enough to produce measurable physical effects. The Casimir effect demonstrates this directly.

Two uncharged metal plates placed extremely close together in a vacuum experience a measurable attractive force not from any known field or charge, but from the difference in quantum vacuum fluctuations inside and outside the gap between them. The vacuum pushes them together. Empty space has pressure. The void is not absence. It is the ground state of reality, the baseline from which everything else emerges, seething with quantum activity, threaded with dark matter, expanding under dark energy, more fundamental than any of the matter floating within it. What we call empty space is the universe's most basic ingredient, the stars of the decoration. Right now, as you prepare to sleep, you are mostly nothing. Not philosophically, not as a metaphor

for insignificance. Physically, measurably, precisely the atoms that constitute your body are almost entirely empty space, and those atoms are themselves arranged into a structure that is overwhelmingly air and void. The matter in your body, compressed to nuclear density, would fit inside a grain of salt. Everything else, the shape of you, the volume of you, the space you occupy in this room, is empty space, held in form by electromagnetic forces between particles that are themselves almost entirely nothing.

You lie down and take up space. You feel solid. You feel present. The bed pushes back against your weight and your skin registers the pressure. and your brain interprets this as physical reality, as substance, as presence, as the unambiguous fact of a body occupying a location. But the pushing back is not solid meeting solid. It is electromagnetic repulsion between electron clouds, the electrons in the atoms of your body repelling the electrons in the atoms of the bed.

Neither set of electrons ever actually touching, The sensation of solidity produced entirely by forces acting across the empty space, inside and between atoms, that are themselves almost entirely empty. You have never touched anything. Not once. Not in your entire life. Every sensation of contact you have ever experienced has been electromagnetic repulsion across atomic scale distances. The universe does not allow matter to touch matter in any direct sense.

The emptiness goes all the way down to every interaction you have ever had with the physical world. And above you, beyond the ceiling, beyond the atmosphere, beyond the moon moving 3.82 centimeters further away tonight, beyond the solar system, beyond the Milky Way, the emptiness continues. Galaxy filaments threading through voids hundreds of millions of light years across. Dark matter halos surrounding structures too large to visualize. Dark energy pushing everything apart from everything else slowly, inexorably, in every direction simultaneously.

The universe is 100% empty. And in the fraction of a fraction of a fraction that is not empty in the extraordinarily improbable concentration of matter that produced stars, that produced planets, that produced chemistry, that produced biology, that produced nervous systems, that produced consciousness, something is lying in the dark right now, aware of the emptiness surrounding it, able to calculate its own insignificance, with the very insignificant matter it is made of. That is not nothing. That is the strangest, most improbable thing the universe has managed to produce in 13.8 billion years of trying. The emptiness is vast.

The awareness of it is rarer. Close your eyes. The void will still be there in the morning. Pick up a handful of sand from a beach. Let it run through your fingers. Count the grains not literally, but imagine counting them. A single handful of dry sand contains somewhere between 5,000 and 10,000 individuals. grains, depending on the coarseness of the sand and the size of your hand. A small bucket, the kind a child brings to the beach, contains perhaps a million. Now scale up. A cubic meter of sand contains approximately 8 billion grains. A stretch of beach one kilometre long, 50 metres wide and two metres deep contains roughly 800 trillion grains. All the beaches on Earth, every coastline, every desert shoreline, every sandy bank of every

river are estimated to contain approximately 7.5 times 10 to the power of 18 grains of sand. That is 7.5 quintillion. A 7 followed by 18 zeros. This number is used so often as a benchmark for incomprehensible quantity that it has almost lost its ability to impress. 7.5 quintillion grains of sand on Earth. We have heard this before. We nod and move on. But here is the number that should stop you. The observable universe contains an estimated 200 billion trillion stars. Written out, that is 2 times 10 to the power of 23, a 2 followed by 23 zeros.

This estimate, refined over decades of astronomical observation and published in a landmark 2016 study in the Astrophysical Journal, it is considered conservative. It accounts only for stars and galaxies we can detect. using detection methods that likely miss significant numbers of faint, small or distant stars. Compare the two numbers directly. 7.5 quintillion grains of sand on Earth, that is 7.5 times 10 to the power of 18. 200 billion trillion stars in the observable universe, that is 2 times 10 to the power of 23. The number of stars exceeds the number of grains of sand on Earth by a factor of approximately 10,000. For every single grain of sand on every beach and desert on Earth, there are roughly 10,000 stars in the observable universe. This is

the comparison that tends to produce a particular kind of silence in people who sit with it properly. Not the silence of confusion, but the silence of something genuinely landing. 10,000 stars per grain of sand. Walk along a beach and pick up a single grain. Turn it in your fingers. That grain represents, in the stars of the observable universe, not one star, not a handful, but 10,000. A private galaxy's worth for each grain. But the comparison extends further than most people take it. The observable universe is not the universe. It is the portion of the universe from which light has had time to reach us since the Big Bang, a sphere of visibility centered on Earth, defined by the speed of light and the age of the cosmos.

Beyond it, the universe continues, in all likelihood, vastly further than the observable portion. If the universe is even moderately larger than the observable portion, The total number of stars it contains dwarfs the observable count by factors that themselves have no meaningful comparison. And stars are not the only objects to count. Many stars have planets. Current estimates suggest the Milky Way alone contains more planets than stars, perhaps several hundred billion planets in our galaxy. The observable universe's 200 billion trillion stars at similar ratios implies something in the range of several hundred billion trillion planets. Moons orbiting those planets add another order of magnitude. Asteroids, comets and

smaller bodies multiply the count further. The observable universe contains more individual rocky bodies than there are grains of sand on Earth, by a factor so large it has no intuitive analogy. And yet... If you took every single one of those stars, all 200 billion trillion of them, and placed them together, the total volume they occupy would be a vanishingly small fraction of the observable universe's total volume.

The stars, for all their number, are islands of matter in an ocean of nothing. The universe is not full of stars. The universe contains stars the way the ocean contains fish present, real. occasionally numerous in local clusters, but utterly overwhelmed by the medium they inhabit. The number is real.

The fullness it implies is not. The universe has more stars than Earth has grains of sand, and it is almost completely empty. Before we measure the emptiness of space, we need to confront something more personal. The matter that fills space, the stars, the planets, the gas clouds, Everything we point to when we say the universe contains something is itself almost entirely empty. Start with the atom. Every piece of matter you have ever touched, every object you have ever seen, every planet and star and galaxy in the observable universe is made of atoms. Atoms are the fundamental units of ordinary matter and atoms are, by volume, almost completely nothing. A hydrogen atom, the simplest and most

abundant atom in the universe, consists of a single proton at its center and a single electron in a cloud around it. The proton has a radius of approximately 0.85 femtometers. The electron cloud extends to a radius of approximately 53 picometers, the Bohr radius. A picometer is 1000 femtometers. The electron orbits at a distance roughly 60,000 times the radius of the proton it orbits. Scale this up to something visible.

If the proton at the centre of a hydrogen atom were the size of a marble roughly one centimetre across the electron, it would be orbiting at a distance of approximately 600 metres away, half a kilometre. The marble sits at the centre of a sphere half a kilometre in radius, And somewhere on the surface of that sphere, a particle far smaller than the marble traces its path through otherwise empty space. Everything between the marble and the distant electron is nothing. No matter. No substance. Just the quantum fields that govern their interaction real in a physical sense, but not matter in any conventional meaning of the word. This is not a peculiarity of hydrogen. Every atom... in every element has the same basic architecture, a tiny dense nucleus

surrounded by an electron cloud that defines the atom's size but contains almost none of its mass, separated from the nucleus by a vast proportional emptiness. The nucleus of any atom contains more than 99.9% of the atom's mass in less than one trillionth of its volume. The rest, the overwhelming majority of the space the atom occupies, is effectively empty.

Your body is made of atoms. Every atom in your body is structured this way. If all the empty space inside every atom in your body were removed, if all the nuclei and electrons were compressed together with no space between them, the remaining matter would fit inside a single grain of salt. Every human being who has ever lived compressed to nuclear density, would fit inside a sugar cube.

But mass is not volume. The question of emptiness is about volume, about how much of the solar system's total space is actually occupied by something rather than nothing. The observable solar system is including the Oort cloud, at its outer boundary, occupies a sphere approximately two light years in diameter.

That is a sphere of roughly 10 to the power of 38 cubic meters of space. The total volume of all the objects in the solar system, every planet, every moon, every asteroid, every known body, adds up to approximately 10 to the power of 27 cubic meters. The Sun alone accounts for the vast majority of that volume. Divide the volume of matter by the volume of the solar system.

The result is approximately one part in 10 to the power of 11, one part in 100 billion. The solar system is roughly 99.999999999% empty space. The matter it contains occupies less than one billionth of one percent of its total volume. To visualize this differently, if you shrunk the solar system to the size of Earth, all the matter in it would be invisible to the naked eye. Every planet, every moon, every asteroid reduced to subatomic scale by the same reduction that brings the solar system down to Earth size.

The space between objects is not incidental to the solar system. It is the solar system. The objects are the incidental part. The asteroid belt is perhaps the most instructive example. In science fiction, asteroid belts are depicted as dense fields of tumbling rocks that spacecraft must carefully navigate a gauntlet of collisions and close calls. The reality is almost the opposite.

The asteroid belt contains hundreds of thousands of asteroids, but they are distributed across a region of space so vast that the average distance between any two asteroids is roughly 600,000 kilometers. Several spacecraft have passed directly through the asteroid belt without any special navigation precautions. the probability of randomly encountering an asteroid while passing through is essentially zero. The solar system is not a crowded place. It is an almost perfectly empty place, with small islands of matter separated by distances so large that travelling between them takes years, even at the speeds of our fastest spacecraft. And the solar system is one of the denser regions of the galaxy. The

solar system is 99.999999999% empty space. That already dismantles any intuition that matter is the dominant feature of the cosmos. But the solar system, for all its emptiness, is a region of extraordinary local density compared to the galaxy it sits inside. The Milky Way contains between 200 and 400 billion stars. It is approximately 100,000 light years across and roughly 1,000 light years thick at the position of our solar system. Its total volume is enormous, approximately 8 times 10 to the power of 60 cubic meters. Now consider the stars inside it. The Sun has a radius of approximately 696,000 kilometers large by human standards, negligible by galactic ones.

The Milky Way is so empty that if you compressed all its stars together, every single one of the 400 billion, the resulting object would be smaller than a grain of sand compared to the galaxy's total size. The average distance between stars and the Milky Way is approximately 4 to 5 light years, roughly the distance to Proxima Centauri. our nearest neighbor. At our scale from the grain of sand video, where the sun is a grain of sand, that distance is 30 kilometers. Two grains of sand, 30 kilometers apart, in a desert that extends 70 million kilometers in every direction.

That is the density of the Milky Way. But stars are not the only matter in the galaxy. The interstellar medium, the gas and dust filling the space between stars, adds mass without adding much volume. On average, the interstellar medium contains approximately one hydrogen atom per cubic centimetre. One atom per cubic centimetre. The best vacuum ever achieved in a laboratory on Earth, 10 to the power of minus 17 atmospheres, contains far more particles per cubic centimeter than the average interstellar space. The space between stars is not merely empty in the everyday sense of the word, it is empty in a way that has no analogy in human experience.

No chamber, no container, no environment ever encountered by any human being has come close to the emptiness of interstellar space. The air in this room contains approximately 10 to the power of 19 molecules per cubic centimetre. Interstellar space contains one atom per cubic centimetre. The Milky Way is a structure defined almost entirely by what it doesn't contain. Its stars are punctuation marks in a sentence made almost entirely of silence. The Milky Way is almost entirely empty. Its 400 billion stars occupy a fraction of its volume so small it has no everyday analogy. And the Milky Way is one of two trillion galaxies in the observable universe.

Now zoom out to the largest scale we can observe. The observable universe has a diameter of approximately 93 billion light years. Its total volume is roughly 4 times 10 to the power of 80 cubic meters. Inside that volume sits all the matter we can detect. Every galaxy, every star, every planet, every atom of gas and dust, in every interstellar medium of every galaxy that has ever existed within our observable horizon. The total amount of ordinary matter in the observable universe, the matter made of protons, neutrons, and electrons that constitute everything we can see and touch is estimated at approximately 10 to the power of 53 kilograms, vast beyond any human scale. But matter is dense. Convert

that mass to volume at average stellar density, and all the ordinary matter in the observable universe occupies approximately 10 to the power of 47 cubic meters. Divide by the universe's total volume. Ordinary matter occupies approximately one part in 10 to the power of 33 of the observable universe.

One part in a billion, trillion, trillion. If you took all the matter in the observable universe, every star, every planet, every gas cloud, every atom of every element... ever forged in every stellar furnace across 13.8 billion years of cosmic history and spread it uniformly through the universe's volume, the average density would be approximately six hydrogen atoms per cubic metre.

Not per cubic centimetre, per cubic metre. Six atoms in a space the size of a large refrigerator. And this is the average. including the relatively dense regions of galaxy clusters and filaments. The universe's large-scale structure is not uniform. It is organized into a cosmic web, a foam-like architecture of filaments and sheets of galaxies surrounding enormous empty regions called voids. The filaments, where galaxy clusters concentrate, are the densest regions. but they are separated by Void's vast spherical regions of space containing almost no galaxies at all. The Boötes Void is one of the largest known, a roughly spherical region approximately 330 million light years across, containing almost nothing.

no galaxies. To put that in context, the entire observable universe is 93 billion light years across. The boat's void is 330 million light years of almost nothing. A bubble of emptiness so large that if the Milky Way were at its centre, we would not have known other galaxies existed until the 1980s. Because the nearest ones would have been at the void's edge. These voids are not rare.

They constitute the majority of the universe's volume. The filaments and walls where galaxies cluster are the exception. The voids are the rule. The universe is not a place filled with galaxies. It is a place filled with nothing, through which thin threads of galaxies are strung like cobwebs across an infinite dark room. Everything we have covered so far points in one direction. Atoms are mostly empty. The solar system is mostly empty. The galaxy is mostly empty. The universe is mostly empty. The voids between galaxy filaments are emptier still. Emptiness, at every scale, is the dominant feature of physical reality. And yet the void is not empty.

This is where the story takes its strangest turn. Because the space between things, the space we have been calling empty, the space that constitutes the overwhelming majority of the universe is not nothing. It is something. Something active, something dynamic, something that physicists are still struggling to fully understand. Start with dark matter. Approximately 27% of the universe's total energy content is dark matter, a substance that does not emit, absorb, or reflect light, interacts with ordinary matter only through gravity, and has never been directly detected despite decades of searching.

Dark matter is not in the stars and galaxies. It is distributed throughout the universe in enormous halos surrounding galaxies and filling the spaces between them. The voids that appear empty to our telescopes are threaded with dark matter invisible, undetectable by any current instrument, but gravitationally present, shaping the large-scale structure of the cosmos. Then there is dark energy.

Approximately 68% of the universe's total energy content is dark energy, the mysterious force driving the accelerating expansion of the universe. Dark energy is not concentrated in objects or structures. It is a property of space itself present everywhere, in every cubic meter of the universe, including the most empty voids, pushing space apart at an accelerating rate. The emptier the region, the more dark energy it contains in absolute terms, because dark energy scales with volume rather than mass. The void between galaxies is, by dark energy content, the richest environment in the universe. Then there is quantum vacuum energy. According to quantum field theory, empty space is not truly empty

at the quantum level. The vacuum is filled with quantum fields, the fundamental entities that give rise to all particles and forces, and these fields are never truly at rest. They fluctuate constantly, producing pairs of virtual particles that pop into existence and annihilate almost instantly. Too brief to detect directly, but real enough to produce measurable physical effects. The Casimir effect demonstrates this directly. Two uncharged metal plates placed extremely close together in a vacuum experience a measurable attractive force not from any known field or charge, but from the difference in quantum vacuum fluctuations inside and outside the gap between them.

The vacuum pushes them together. Empty space has pressure. The void is not absence. It is the ground state of reality, the baseline from which everything else emerges, seething with quantum activity, threaded with dark matter, expanding under dark energy, more fundamental than any of the matter floating within it. What we call empty space is the universe's most basic ingredient, the stars of the decoration. Right now, as you prepare to sleep, you are mostly nothing. Not philosophically, not as a metaphor for insignificance. Physically, measurably, precisely the atoms that constitute your body are almost entirely empty space, and those atoms are themselves arranged into a structure that is overwhelmingly air and void.

The matter in your body, compressed to nuclear density, would fit inside a grain of salt. Everything else, the shape of you, the volume of you, the space you occupy in this room, is empty space, held in form by electromagnetic forces between particles that are themselves almost entirely nothing. You lie down and take up space. You feel solid.

You feel present. The bed pushes back against your weight and your skin registers the pressure. and your brain interprets this as physical reality, as substance, as presence, as the unambiguous fact of a body occupying a location. But the pushing back is not solid meeting solid. It is electromagnetic repulsion between electron clouds, the electrons in the atoms of your body, repelling the electrons in the atoms of the bed.

The awareness of it is rarer. Close your eyes. The void will still be there in the mornings. Thanks for watching! Thank you. Thank you for watching. Thank you. Thank you. Thank you for watching. Thank you. Thank you. Thank you. Thank you. Thank you for watching. Thank you. Thank you. Thank you. Thank you. Thank you for watching. Thank you.

Thank you. Thank you. Thanks for watching!

There Are More Stars Than Grains Of Sand _ But The Universe Is Almost Completely Empty

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