How the Universe Works - S05E08 - The Dark Matter Enigma
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00:00Everything we see, from people, to cities, to the millions of stars that fill the night
00:09sky, they're all made of the same stuff, atoms of matter.
00:16But there's something else out there, something weird, invisible dark matter.
00:22It fills our universe too, and it could be the key to the existence of everything, including
00:30us.
00:46There's something about our universe that might surprise you.
00:51We exist side-by-side with cosmic ghosts.
00:57Normal matter, the stuff that makes stars, planets, and people, accounts for just a fraction
01:02of what's really out there.
01:07An invisible web of strange material connects the galaxies that make up our universe.
01:13We can't see or feel this mysterious stuff, but we're pretty sure it's there because we
01:19can measure its gravity.
01:21It's a little weird to discover that what you see, what you know, everything in your
01:28experience is actually the tiniest, teeniest fraction of what's actually out there in the
01:33universe.
01:36Most of the matter in the universe is made of a substance that we have not even discovered
01:41yet.
01:46This strange, ghostly stuff is called dark matter.
01:52Most scientists believe dark matter is a kind of elementary particle hanging in vast networks
01:59of clouds or halos that link the two trillion galaxies that make up our universe.
02:07If you could put on some sort of mask or goggles that could detect dark matter, you would see
02:12millions of them passing through you every second, millions of them, billions all around
02:17you all the time.
02:19You can't see them, but their effect is very real.
02:23Crucially, we are now beginning to understand that ghostly tendrils of dark matter have
02:30shaped everything we see in our universe today.
02:35They may have even played a key role in the evolution of us.
02:41Dark matter is kind of like an invisible puppet master with invisible strings controlling
02:46the movement of everything we can see.
02:49It is the central reason for our existence.
02:53The fact that our galaxy exists for us to exist in is due to the fact that dark matter
02:58exists.
02:59If you were to think about it, dark matter is the matter that matters.
03:04So how did dark matter shape the universe we see today?
03:08Best way to find out?
03:10Take a trip back to the beginning of everything, the Big Bang.
03:1513.8 billion years ago, an infinitely hot and infinitely dense speck bursts into existence.
03:27This speck is the infant universe.
03:30It's hot and filled with nothing but pure energy.
03:36As it expands, it cools, and some of the energy condenses to form tiny subatomic particles.
03:44But these aren't the protons or electrons that make up you and me.
03:48They're particles of dark matter.
03:53If dark matter is made up of strange subatomic particles, these were probably created in
03:58the very, very early universe, moments after the Big Bang itself.
04:03And normal matter may not have existed yet when dark matter did.
04:09The universe is still less than a second old.
04:13It's incredibly dense and hot.
04:16In this confined space, the dark matter particles are crammed tightly together.
04:22Collisions are inevitable.
04:25The particles annihilate each other as they smash together, releasing a burst of energy.
04:33Plus something new.
04:36Subatomic particles of ordinary matter, the stuff that makes up the universe we can see.
04:45It's very plausible that two dark matter particles that collided and annihilated in the very
04:49early universe produced an electron that's now part of my body.
04:54So I might actually be a child of dark matter, even in a very direct and literal sense.
05:01Did colliding dark matter particles really make all the ordinary matter we see in the
05:05universe today?
05:08It depends on what dark matter particles are made of.
05:13The best bet is that dark matter is a particle 100 times the mass of a proton.
05:20But unlike ordinary matter, it doesn't interact with light or anything else.
05:27One of the top contenders for dark matter is something called a WIMP, a weakly interacting
05:31massive particle.
05:32This is a particle that was made in the Big Bang that's left over today.
05:38WIMPs are the leading contender because if you plug their properties into computer simulations
05:43of the Big Bang, you end up with a universe that looks just like the universe we see today.
05:50With 84% dark matter and just 16% ordinary matter.
05:57We get numbers that correspond roughly to the amount of dark matter we infer in the
06:03universe.
06:04So there's good evidence, indirectly, that these particles may be the dark matter.
06:12If current thinking is right, the subatomic building blocks of the universe were forged
06:16from colliding WIMPs.
06:19But dark matter's role in building the cosmos was just getting started.
06:23In fact, dark matter may answer one of the great mysteries of cosmology, how the primordial
06:30gas that filled the early universe clumped together to form the first stars.
06:36The mystery begins when the universe is less than a second old.
06:42It suddenly expands.
06:45In a period of about a millionth of a billionth of a billionth of a billionth of a second,
06:50our universe puffed up by over 90 orders of magnitude in volume and went from the size
06:56of a single atom to the size of a basketball in a fraction of a fraction of a fraction
07:01of a second.
07:04This rapid expansion creates a vast sea of evenly spread particles which cool to form
07:11atoms of hydrogen and helium.
07:15The gases that will one day collapse under the force of gravity to become the first stars.
07:22But there's a problem.
07:25The gas of the early universe is too evenly spread, too smooth for gravity to pull on
07:31some parts more than others and trigger regions of the gas to collapse and clump.
07:37If the universe was completely smooth, it would be beautiful but boring because nothing
07:43would exist that we could see.
07:48Something must have made the smooth sea of gas collapse and build the first stars.
07:55Something weird operating on the tiniest of scales.
08:01One thing that really is interesting about that is that on very small scales due to the
08:05Heisenberg uncertainty principle, strange things can happen.
08:11When we see a car, a runner, or even a spacecraft, we can calculate their motions.
08:19In the tiny quantum world of the infant universe, that certainty is missing.
08:26Nothing has a definite momentum or position and because nothing was locked in place, fluctuations
08:31or grooves could develop in the expanding universe.
08:36And when the universe inflated rapidly, these fluctuations became frozen in place, creating
08:42dense points around which the gas clouds could collapse, acting like gravitational seeds
08:49for star formation.
08:51Fortunately, there were these tiny seed fluctuations which acted like a kind of cosmic DNA determining
08:59where and when and how structure later grew into the stars, the planets, and all the other
09:04awesome structure we see around us in today's world.
09:08I think it's one of the most beautiful ideas of all of science that something like the
09:12Heisenberg uncertainty principle, which we thought applied only to tiny things in quantum
09:16mechanics, ultimately is responsible for the biggest structures that we know of in the
09:21cosmos.
09:22So, thank you, Heisenberg.
09:30The development of the fluctuations seems to solve the mystery of how the universe evolved
09:35its structure.
09:37But there's another problem.
09:39If you do the math, the mass of gas alone doesn't pack enough gravity to create all
09:45the stars we see in the universe today.
09:50Something else must have added mass to the collapsing gas clouds.
09:55Could that something have been dark matter?
10:12Today cosmologists are grappling with the puzzling paradox.
10:16How did the gas that once filled the universe collapse so quickly to form the stars we see
10:21today when there wasn't enough gas to begin with?
10:25The only answer?
10:27Something other than normal matter must have been out there, adding mass to the gas clouds,
10:32helping them collapse into stars.
10:36If you only have the normal matter, it turns out things just don't grow fast enough.
10:40You don't have enough structure in the universe.
10:43We can calculate that there wouldn't have been enough time since the beginning of the
10:47universe for normal matter to collapse to form galaxies, stars, planets, and people.
10:54Many scientists now believe the extra push speeding up the formation of stars was the
10:59gravity of invisible dark matter.
11:03Even though dark matter and normal matter can't really interact directly, they do interact
11:09via gravity.
11:10And it turns out that is critically important to our existence.
11:14If you put that dark matter in, everything works out, and it's really kind of amazing
11:18how well we can make the universe work.
11:24As the early universe expands, it also cools.
11:29It's now a sea of hydrogen and helium gas.
11:36There's also lots of dark matter around, which has built up in the fluctuations or grooves
11:42in the expanding universe, creating regions of high gravity.
11:48The dark matter was free to actually start doing its own thing and start growing its
11:53patterns and its clustering before the ordinary matter did.
11:56And that's why dark matter actually played such a key role in creating this much more
12:01interesting universe that we live in today.
12:05The gravitational pull of these clumps of dark matter drags in huge clouds of hydrogen
12:11and helium.
12:14The clouds get denser and denser until they trigger nuclear fusion, and the first stars
12:22in our universe are born, thanks to dark matter.
12:29It clumped and collapsed, and that would later allow all the normal matter to fall in.
12:37Dark matter is what gave the initial kick to form stars, black holes, planets, aliens,
12:42people, and everything else.
12:50You really have to understand that dark matter is the dominant form of matter in the universe.
12:55At the very beginning of the universe, that's what got everything started, and regular matter
13:00was just along for the ride.
13:05Dark matter explains how the first stars in the universe burst into life.
13:12But when astronomers gaze back to the early universe, they see these stars weren't alone.
13:19They lived alongside monsters, supermassive black holes.
13:25A real puzzle is that we see some of these supermassive black holes in the very early
13:30universe.
13:31So there really wasn't enough time between the Big Bang and when we're studying these
13:34things for them to grow to such large sizes.
13:41Supermassive black holes are the heavyweights of the early universe.
13:46Some weigh in at 12 billion times the mass of our sun.
13:51How they grew so gigantic so quickly has been one of the biggest mysteries in cosmology,
13:58until perhaps now.
14:03Some scientists believe the beginnings of these early black holes could have been formed
14:08by a strange superstar called a dark star.
14:13Dark stars would be the very first stars to form in the universe.
14:19So they form when the universe is about 200 million years old.
14:23These are very early objects.
14:25So they are made of ordinary matter.
14:27They're made of hydrogen and helium, but they're powered by dark matter.
14:34Catherine Friess believes that as these giant early stars formed in the early universe,
14:41their enormous gravity dragged dark matter particles into their cores.
14:49These particles smashed into each other, releasing bursts of energy.
14:55Whenever they encounter each other, they annihilate and turn into something else.
15:01That means a lot of heat is released, a lot of energy, and it's that energy that could
15:05power stars.
15:10So it's possible that in some stars, their internal reactions are actually being powered
15:15by dark matter.
15:17Effectively, dark matter annihilation is providing energy to keep these stars lit up.
15:27It's quite remarkable because you only need one part in 10,000 of dark matter to power
15:32an entire giant star.
15:38The energy from dark matter annihilations allowed the dark stars to become super large.
15:46These early objects are really strange.
15:48They're very cool, and they're really, really big.
15:53The size of these things is 10 times the size, the distance between the sun and the earth.
15:59So they're really, really big, puffy, cool, and powered by dark matter annihilation.
16:07But when their dark matter fuel ran out, these huge stars had nothing left to hold them up.
16:14There's nothing to sustain this big, puffy object.
16:17It's going to collapse.
16:19If it's big enough, you collapse directly to a black hole.
16:24Because the collapsing star was so huge, the new black hole it formed was also supermassive.
16:33If dark stars exist, then black holes could be born big.
16:38So because of dark matter, you could start with very, very massive black holes early
16:42in the universe.
16:44But that's only because you have the dark matter to power these early stars.
16:49Dark matter may have helped form the basic units of our universe, stars and black holes.
16:58But did the reach of this mysterious puppet master stretch even further?
17:04There are patterns in the distribution of galaxies and galaxy clusters that normal matter
17:10alone can't explain.
17:25Our home star, the sun, is just one of 200 billion stars that make up our galaxy.
17:33But how did this vast collection of stars first form?
17:38Do the Milky Way with imaginary dark matter goggles and you'll find a clue, a halo of
17:47dark matter surrounding the galaxy.
17:51A dark matter halo helps a galaxy form simply by providing gravity to pull things together,
17:59catalyzing it.
18:00And that may have allowed our galaxy to form.
18:02The gravity from this huge construct brought regular matter in to form the Milky Way in
18:07the middle.
18:11Astronomers used to think the distribution of galaxies throughout the universe was random.
18:17But recent observations have discovered something extraordinary.
18:22Walls of interlinked galaxies that stretch through space for millions and millions of
18:28light years.
18:29One of the most amazing discoveries of the last few decades is that galaxies form these
18:35vast superstructures that actually span the known universe.
18:38The Great Wall is a filament of galaxies that stretches hundreds of millions of light years.
18:46To understand how these vast galactic structures formed, astronomers used telescopes like the
18:52Atacama Large Millimeter Array, or ALMA for short.
18:59It's an array of 66 radio telescopes and so powerful, it can peer back billions of
19:06years to the formation of the earliest galactic structures.
19:11Now ALMA can actually look back into the history of the universe and see similar structures
19:16being formed by these giant baby galaxies, sort of a proto-Great Wall.
19:25ALMA gives us a snapshot of how the adolescent universe evolved.
19:31It shows us that as the universe expanded, the newborn galaxies aligned with their neighbors.
19:39It was as if the expanding universe was producing sticky filaments and the materials building
19:45new galaxies were sticking to these threads like flies to a spider's silk.
19:52Turns out these invisible filaments are dark matter.
19:57It's like a scaffolding of dark matter that was pulling normal matter into it.
20:06As the universe expanded, the original clumps of dark matter didn't expand as quickly as
20:11the rest.
20:13They stuck together like sticky taffy with their powerful gravity shaping them into filaments.
20:20The filaments formed a sprawling web of dark matter strands stretching throughout the universe.
20:29The gravity of this dark matter web then dragged in normal matter, which built up where the
20:35filaments meet and eventually collapsed to form galaxies.
20:42The thicker filaments pulled in the most gas, providing the building blocks for galaxy clusters.
20:50They actually fall along these tremendous filaments across the universe, hundreds of
20:55millions of light years across.
20:57We're talking about tremendously large structures, but they would not exist if it weren't for
21:02dark matter.
21:03And the galaxies themselves were able to form because of this structure.
21:08There are galaxies and stars and planets and you here today.
21:12That's because of the dark matter providing the framework.
21:18Just like a grid system in a city defines where buildings are going to be, galaxies
21:23assemble themselves around the cosmic grid.
21:27It seems like city planners here on Earth have been following the lead of the universe,
21:32except those planners used roads in place of dark matter.
21:37New York is the perfect example.
21:40Let's imagine New York City without roads.
21:44There would be no structure, no foundation, and the whole thing would fall apart.
21:50When people first designed the city, they laid down the grid, they built the roads,
21:55and that was really the foundation.
21:58And then later on, they built the buildings.
22:02Just like the dark matter web transported the building materials for galaxies, New York's
22:07grid of roads brought the steel and concrete to build its city blocks.
22:13It's the dark matter that gives you the foundation, gives you the cosmic structure, and then later
22:19on, the normal matter fell into the galaxies and the clusters that we see today.
22:26The elegance of this newly discovered structure of the universe really astounds me.
22:30You have this web of dark matter, and it almost creates highways for regular matter to fall
22:35into these nexuses, and that's where you form the biggest, brightest galaxies.
22:44The more cosmologists study dark matter, the more they see the crucial role it's played
22:49in shaping the universe we see today.
22:53But despite these insights into our origins, scientists still don't know what dark matter
23:00actually is.
23:03How do you measure something?
23:05Something that you can't see or feel.
23:22In the last 50 years, astronomers have discovered something incredible about our universe.
23:29It's controlled by an invisible puppet master called dark matter.
23:37Dark matter created and organized the large-scale structure of our universe.
23:44It may have even created the atoms that make up your body.
23:50But it remains a mystery because we can't see it, feel it, or measure it directly.
23:59One of the properties of dark matter that we know for sure is that it doesn't interact
24:03with light.
24:04It doesn't shine, it doesn't reflect light, and so if you shine a laser beam on dark matter,
24:09the laser beam goes right through, it doesn't do anything.
24:14The term dark matter might actually be a little bit misleading.
24:17Maybe the real term for this stuff is transparent matter, but somehow that doesn't sound quite
24:21as cool as dark matter.
24:24So if we can't see dark matter, why are scientists so sure it's there?
24:31We know dark matter's there the same way we know many things are there that we can't see.
24:35We use gravity.
24:36Dark matter may not interact with light, but it interacts with gravity.
24:41It's only by studying the motions of stars, even the motions of galaxies and the clustering
24:46of galaxies, do we know that there's a lot of extra stuff out there that has mass that's
24:51directing these stars and galaxies to move in ways that are somewhat unexpected.
24:58The first person to detect this unusual movement in galaxies was astronomer Fritz Zwicky in
25:04the 1930s.
25:07Using a custom-built 18-inch telescope, he studied how multiple galaxies interact inside
25:14a tight grouping of galaxies known as a cluster.
25:19Fritz Zwicky studied the motions of galaxies in clusters, and what he saw is that these
25:24galaxies were moving too fast.
25:28Every concentration of matter has associated with it an escape velocity, and if you're
25:33moving faster than the escape velocity, you should no longer be a part of that system.
25:40Just as a rocket can escape Earth's gravity if it's traveling fast enough, a galaxy should
25:47break away from a galaxy cluster if it's moving with enough speed.
25:52Zwicky noticed that the galaxies in the coma cluster should be moving fast enough to escape
25:58the gravitational pull of their neighbors.
26:01But instead of speeding off into different parts of the universe, the galaxies remained
26:06bound together.
26:12He thought some unknown material was adding mass and therefore extra gravity to the system,
26:19holding the galaxies in place.
26:27Maybe there's something extra there that's providing more gravity than we can account
26:32for based on what we see.
26:34At the time, Zwicky's ideas for this unexplained source of extra gravity fell on deaf ears.
26:43Zwicky was a very unlikable fellow and I think infuriated many of his colleagues, and that
26:49perhaps was one other reason they were less willing to accept his suggestion.
26:54The other is that I think he was so far ahead of his time as a scientist that it just took
26:59time for the rest of the community to catch up.
27:05It took another 30 years and a crucial discovery by astronomer Vera Rubin before dark matter
27:12made it into the textbooks.
27:16Vera Rubin was looking at galaxies themselves.
27:19Now galaxies are collections of gas and stars and dust and they have an overall motion.
27:24Our Milky Way galaxy is a disk and that disk is moving around the center of the galaxy.
27:30The stars in the middle of the galaxy you expect to go around very fast and the stars
27:33at the outskirts of the galaxy you expect to go around very slowly.
27:37Just like the inner planets go around the sun very quickly and the outer planets go
27:40around the sun much more slowly.
27:43What she found is that the outskirts of a galaxy were spinning around the galaxy at
27:47the same speed as parts that were closer in and that didn't make any sense.
27:53The stars of the galaxy appeared to be fixed almost as if they were glued to a giant spinning
27:59wheel.
28:03And the only way that can be is if there's some additional gravity there.
28:06Some additional stuff there that's adding mass, that's adding gravity, that's making
28:10those outer stars go faster.
28:14The only conclusion was that the spinning wheel was being affected by something very
28:20massive and completely invisible.
28:24When astronomers turned their telescopes to see for themselves if this invisible mass
28:29was real, they found evidence for it almost everywhere they looked.
28:39As more and more other people were able to replicate what they had done, people started
28:43to realize, whoa, you know, Fritz and Vera were right.
28:47This weird stuff is really out there.
28:50But there's not only just a little bit.
28:54Every galaxy we can see, essentially, is dominated by dark matter.
29:01Now we know that dark matter is out there, but we're still no closer to working out what
29:07it actually is.
29:10We think it may be a particle called a WIMP, but it could be a whole family of particles
29:17that forms dark atoms and dark molecules.
29:21Perhaps an entire dark universe is out there, filled with invisible, dark planets, and illuminated
29:29by the dark light of their dark stars.
29:47Scientists are struggling to uncover the true identity of dark matter.
29:52To date, the best candidate is a theoretical particle known as a WIMP.
29:59But because WIMPs can pass through ordinary matter like a ghost, the researchers have
30:04been left empty-handed.
30:11Not only could dark matter be going through me right now, but it almost certainly is,
30:17and I'm just not noticing it.
30:22This is why we're building these very fancy detectors here on Earth, to try to catch them.
30:31Until scientists can capture and analyze a WIMP, all bets are off.
30:39The main challenge for the WIMP theory is that we simply haven't found any WIMPs yet,
30:44and we've been looking pretty hard for a lot of years, and pretty soon it's going to start
30:48to get embarrassing.
30:52Perhaps dark matter isn't made of WIMPs. Perhaps the stuff that makes up dark matter is stranger
30:58and more complex than we ever thought possible.
31:02We know that regular matter comes in many different forms. There are electrons and protons,
31:06neutrons, quarks, all of those.
31:10Why should we assume there's only one kind of dark matter?
31:13Our approach could be all wrong. Instead of looking for a single type of particle, there
31:17could be an entire zoo of dark matter particles.
31:23Particles of ordinary matter interact with each other to form atoms and molecules, the
31:28stuff we touch and see.
31:32If dark matter is made from different particles, it could do the same, interacting and building
31:39dark atoms of dark stuff. Perhaps even a universe of dark materials.
31:50If we can show that dark matter interacts with itself, that means there really could
31:55be dark matter galaxies, dark matter stars, dark matter planets and people all around
32:00us right now that we were not aware of.
32:05Could this shadowy, dark universe really exist?
32:14In 2012, the Chandra X-ray Telescope gave astronomers the first clue to whether dark
32:20matter interacts.
32:25The telescope observed the collision of two galaxy clusters, each packed with hundreds
32:31of galaxies.
32:38Astronomers hoped to see what would happen to the dark matter inside the clusters. Would
32:43it show any signs of interacting?
32:47What does the dark matter do when these clusters collide? Well, that's the big question. When
32:53the clusters come together, does the dark matter smack and drag, or does it go right
32:57on through? If we can measure that difference, we can tell, does the dark matter self-interact
33:02or not?
33:06The galaxies pass through each other. But something is left behind. A tangle of dark
33:16matter.
33:19It looks like there's some extra dark matter that's been left behind in the middle. So
33:23it looks like as these two dark matter balls have come together, there's been some extra
33:27drag. And that drag has deposited, has left a bit of dark matter kind of sitting in between.
33:36To produce the drag, the dark matter in the colliding galaxy clusters must have interacted.
33:44If this observation turns out to be correct, it means that some dark matter interacts with
33:50itself.
33:53If that signal holds up, then it could be a really smoking gun detection of self-interacting
33:59dark matter.
34:01And that would be a game changer.
34:05Perhaps there could be dark matter planets and dark matter living entities.
34:10There could be a hidden dark matter universe, if you wish, of hidden dark matter objects.
34:16It stretches the limit of plausibility, but it's not impossible.
34:23If this dark sector really exists, there could even be dark light.
34:29So imagine you have this dark matter universe, and it has this dark radiation, and this dark
34:35radiation can travel in waves that we call dark light. Then maybe you can put on your
34:39dark glasses and actually view this dark universe via the dark light.
34:48Interacting dark matter suggests invisible worlds next to our own.
34:54But are these visions real, or just a sci-fi fantasy?
35:01I think dark matter stars, dark matter planets, dark matter people are more in the realm of
35:06science fiction at this point.
35:10It would require a plethora of tooth fairies to imagine that the dark sector is that complicated
35:16to actually reproduce something like our sector. So in order to have dark planets and dark
35:21people and dark TV shows, people have imagined it. I'm not saying they haven't, but it certainly
35:28stretches the realm of credibility.
35:32There is so much dark matter out there that it controls the very fate of our universe.
35:36If even a tiny fraction of it can interact with other forms of dark matter, then maybe
35:41all bets are off.
35:44Even a tiny percentage of dark matter interacting could have huge implications for the future
35:50of life on Earth.
35:54Because a controversial new idea suggests that, in the future, self-interacting dark
36:00matter could send a hail of comets our way.
36:05How do we know? Because dark matter may have done it before.
36:12Dark matter is the dominant creative force in our universe. But could it have helped
36:32to create us, too?
36:36Some scientists believe that a key moment in human evolution may have been directly
36:42affected by dark matter in the Milky Way.
36:51This controversial theory is inspired by one of the most violent days in Earth's history.
36:5965 million years ago, an object the size of Mount Everest slammed into the Earth.
37:12The impact turned the sky black and set the continents on fire.
37:19The amount of energy released in that explosion is about a billion times the Hiroshima atom
37:26bomb that was dropped in World War II. A billion times.
37:37This catastrophic event ended the age of dinosaurs and paved the way for our mammalian ancestors
37:44to flourish. But was it a one-off?
37:48Mass extinctions have happened on Earth multiple times. So an obvious question to ask, is there
37:53any pattern? Is there any time that we know is more dangerous, more likely to have a mass
37:58extinction event? And is there anything we can link it to in the larger universe?
38:05Geologist Mike Rapinoe thinks there is a pattern. And it could be linked to dark matter.
38:11We were looking at the possibility of cycles in the geological record. And we found what
38:16seemed to be a 30 million year cycle in many kinds of geological phenomena. Now what could
38:21be causing this kind of a cycle of 30 million years?
38:25A clue comes from the passage of the sun around the Milky Way.
38:32It takes the sun about a quarter billion years to move once around the Milky Way galaxy.
38:37But as we spin around the galaxy's center, we also kind of bob up and down through the
38:42larger disk of the galaxy.
38:46Breathing like a carousel horse, our sun passes through the galactic plane around once
38:52every 30 million years. And as it does this, it may also be passing through a layer of
38:59self-interacting dark matter.
39:04It's a little bit speculative, but the idea is that a very small fraction of the dark
39:08matter is able to form a disk, along with the disk of our galaxy. So in between the
39:14stars, in the disk where the sun lives, there's a very, very thin disk of dark matter. And
39:19this could do very exciting things.
39:26This disk has a strong gravitational pull. When our solar system passes through it, the
39:34disk's gravity may disrupt the orbits of comets in the outer solar system and send them hurtling
39:42towards the Earth. Perhaps the object that killed the dinosaurs and paved the way for
39:52human evolution was just one of a series of extinction-level events triggered by dark
39:59matter.
40:00We call this the Shiva hypothesis, because the god Shiva in the Hindu religion is the
40:05god of destruction and renewal. So one world is destroyed, the dinosaur world is destroyed,
40:12and the world of mammals and birds begins.
40:15If that's true, then dark matter has played an important role in us being here from the
40:21very beginning, right after the Big Bang, all the way through the evolution of humans
40:27themselves.
40:31The idea that dark matter caused mass extinctions on Earth is, frankly, terrifying. The solar
40:38system is still bobbing up and down through the galactic plane. Perhaps the next disrupted
40:45comet will have our name on it.
40:50Fortunately for us, the theory that dark matter helped wipe out the dinosaurs is just that.
40:59We are talking about speculation on top of speculation. If dark matter exists in this
41:05matter, if it's made of some sort of particle that interacts with itself just enough that
41:10it forms a disk, and if this disk exists, if, if, if, if, this idea is interesting.
41:15If I were a bookmaker here in New York, I would bet against dark matter killing the
41:20dinosaurs, but I think it's great that people are doing the math more carefully and really
41:26testing these scenarios.
41:33The outlook is unclear, but one thing's for sure. Without dark matter, we wouldn't even
41:40be here.
41:43The fact that there's a universe full of matter and stars and galaxies is due to the fact
41:48that dark matter exists.
41:51So if some young person in some ways asked me where do I come from, I'd have to say
41:58from dark matter.
42:02Dark matter dictates how the galaxies form and how they move and where we'll go in the
42:05future. We're actually in dark matter's universe, not ours.