How the Universe Works - S10E03 - Hunt for Dark Matter
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00:00Scientists believe there is a hidden substance deep in space that keeps the cosmos running.
00:08But is that substance real?
00:12We've never seen dark matter. It's completely invisible, but we know that it has to be there.
00:18Not only can you not see it, you couldn't really touch it or taste it or smell it.
00:23And yet it is all around us. It affects everything that we do.
00:26After searching for decades, we still don't understand this inexplicable substance.
00:33We know dark matter is there because we feel its strong gravitational pull, but it just doesn't want to talk to us.
00:40There's evidence that dark matter makes up 85% of all the matter in the universe.
00:46We can see dark matter holding galaxies together and ripping other structures apart.
00:52We even see it bending light.
00:55Dark matter itself has been around since the beginning of the universe.
00:59Without dark matter, we wouldn't be here.
01:03But if you can't see dark matter, and if you can't touch it, does it really exist?
01:10The Hyades Star Cluster.
01:15This family of 700 stars is 150 light years from Earth.
01:21At the scale of the universe, it's in our backyard.
01:26Hyades is actually closer to the Sun than it is to the Earth.
01:32When you look up at the night sky, Hyades is in that V-shape in Taurus the Bull.
01:38For most of its 650 million year lifetime, the Hyades enjoyed a peaceful existence.
01:45But something is breaking the calm.
01:49The Hyades Cluster is one of the most well-structured clusters in the universe.
01:55But something is breaking the calm.
01:59The Hyades Cluster is one of the most well-studied clusters of stars we have in the entire sky.
02:04And yet there's something very deeply mysterious going on.
02:08Two star tails extend from the cluster's center.
02:12They should be roughly equal, but one tail is hemorrhaging stars.
02:18Something is disrupting it. There's something exerting a force on it that's ripping stars out of their orbits.
02:26Something with immense gravitational pull has passed by the cluster and robbed it of stars.
02:34In order to be gravitationally pulling stars out of an object like Hyades,
02:39you need to have an incredibly massive structure, as much as 10 million times the mass of the Sun.
02:45This monstrous cosmic mugger should still be visible.
02:49But when we point our telescopes to where it should be, that region is empty.
02:57There is nothing there, and I mean nothing.
03:01And not a little bit or something dark or something small, but there's literally nothing that we can see.
03:08We know something is out there, invisible and powerful.
03:13And whenever we witness these unseen assaults, a prime suspect gets called in.
03:19A phantom of physics.
03:22Dark matter.
03:25So what can we confidently say about this mysterious cosmic substance?
03:32It does not emit light. It does not reflect light. It does not absorb light.
03:37The only thing we know about dark matter is that it has gravity.
03:41We're not even really sure it's matter at all. It's just that that's the only thing we know that has gravity.
03:46We may not be able to see or touch dark matter,
03:50but we are very good at finding its fingerprints all over the universe.
03:55We can see dark matter's use of gravity to break and bind structures,
04:01and we've been spotting its handiwork for decades.
04:05Let's rewind back to 1933.
04:10Swiss-American physicist Fritz Zwicky tracks strange movements
04:15in a far-off collection of galaxies called the Coma Cluster.
04:20He knows he's not seeing the whole picture.
04:24Some galaxies are speeding around the cluster at inexplicably fast rates.
04:30Zwicky's looking at these galaxies, and if the only mass that was there were the other galaxies you can see,
04:35you would expect these galaxies to be moving at about 50 miles a second.
04:39Then they would stay bound to each other and not fly apart.
04:42Instead, he sees them moving at 1,000 miles a second.
04:47At these velocities, galaxies should be flying off the cluster like sparks from fireworks.
04:55Zwicky realized there had to be extra stuff.
05:00In his words, dunkle materia.
05:03Dark matter.
05:05Dark matter.
05:06Dark matter.
05:08It becomes clear that Zwicky's Coma Cluster isn't an isolated case.
05:15Astronomers begin seeing the same dynamics within galaxies themselves.
05:21In systems governed exclusively by gravity, objects farthest away from the center
05:26would take the longest to complete an orbit.
05:29But in many galaxies, stars on the outside are orbiting at almost the same rate as those in the core.
05:37It's almost like a photograph record.
05:39Every part of that record spins around like a solid disk.
05:43The stars are going too fast to stay bound to the gravity of the galaxy.
05:47They should just fly right off into space.
05:51Physicists come up with an explanation.
05:54Galaxies sit in a giant halo, or ball, of invisible dark matter.
06:00And it's that extra mass that allows the stars to turn fast all the way out to the galactic rim.
06:07Think about actually taking a disk of dough and spinning it to make a pizza.
06:11The more you spin it, the more those outer regions go farther and farther away.
06:16Eventually the dough just goes flying everywhere.
06:19That's what would happen to a galaxy if it weren't for dark matter.
06:22But as you spin pizza dough, and you spin it faster and faster, it does hold itself together
06:27because there's all this yummy gluten that's acting as a glue.
06:31Dark matter is the gluten of our universe.
06:35By calculating the mass needed to bind those speeding outer stars to the galaxy,
06:41physicists are able to estimate how much visible matter there is compared to dark matter.
06:48The results are staggering.
06:50All the stuff we thought existed was just maybe 15% of our universe.
06:55That's like if you go to a restaurant and realize there's a measly 15% tip.
06:59You know, that's all we are.
07:01We're not even the majority substance.
07:04We may not be able to see it, but dark matter makes up some 85% of all matter.
07:10Wherever we look, we can see its gravity having effects.
07:15It glues galaxies like our Milky Way together.
07:19And a close look reveals dark matter can also bend light itself.
07:27It's called gravitational lensing.
07:30A massive object can bend space and time, and light must follow the curves of that space and time.
07:39Gigantic clumps of any matter create a gravitational lens.
07:45Dark matter showed its space-warping power in a trick it played with a gigantic explosion in a far-off galaxy cluster.
07:55Supernova Repsdal was first detected in November of 2014.
08:02Supernova Repsdal is actually one of my favorite recent results in all of the astronomical literature.
08:08That result blew me away.
08:10So a star explodes. Light is emitted in all directions, and some of it makes its way towards the Earth.
08:16So far, so good. This is very standard.
08:19So the flash appears, and then another flash appears.
08:25We see it again, and again, and again.
08:30We see the explosion go off in four parts of the sky, and then a year later, a fifth explosion goes off in a totally different part of the sky.
08:39What's going on?
08:41Analysis proves that these multiple explosions are the same supernova.
08:46But between this one dying star and our telescopes sits a giant mass of dark matter, a huge gravitational lens.
08:58What that means is that some of these rays of light will take much longer, more complicated paths through this region of space-time.
09:07The dark matter lens turns one supernova into a fireworks display, lasting an entire year.
09:16Dark matter affected the trajectory of light from this supernova so much that for some of those trajectories, it added a whole light year.
09:25It took a whole extra year for light to reach us.
09:30Something is very definitely out there, distorting our view of the cosmos.
09:37It's a potent clue that dark matter is real.
09:42Now, new evidence suggests that without it, we might not exist at all.
09:49The Cosmos is filled with an unseen substance.
09:53Its mass even bends starlight.
09:57Gravitational lensing suggests dark matter holds our entire universe together.
10:04For decades, this specter of space has haunted us.
10:09We don't know why.
10:12For decades, this specter of space has haunted us.
10:17We've never been able to pin it down.
10:20In 2021, an international team ran a virtual experiment to try to predict where dark matter should be by letting computers map out where we think it lives.
10:32Because we think we know how it behaves, we can model what it should be doing in supercomputer simulations.
10:40The team taught the computer how dark matter bends light, then applied computational power to 17,000 unexplored galaxies.
10:53The model created a dark matter map.
10:57I think a lot of people, when they imagine the universe on the largest scales, think it's sort of boring, everything's uniform.
11:03But that's not what we see.
11:06What's amazing is that on the largest scales of the universe, we see a very particular pattern.
11:12When we zoom out, we see this magnificent structure, this cosmic web that's created by dark matter.
11:19The interweaving tendrils of dark matter stretch for thousands of light years across the cosmos.
11:26At the junctions, where matter is concentrated, we find galaxies form, illuminating the dark scaffold.
11:35If dark matter exists, scientists believe it makes up 85% of the matter in the universe and also controls the remaining 15% regular matter, like stars, planets, us.
11:53If they're right, dark matter played a critical role in actually building the universe we see today.
12:052021. Astronomers using the SkyMapper observatory in Australia trained specialist optics on a dwarf galaxy called Toucana 2.
12:17The SkyMapper's filters split up the starlight into a spectrum of wavelengths, revealing some very ancient light.
12:28One of the best clocks that we can put on the universe is the progress of chemistry, right?
12:35The buildup of more complex elements over time.
12:40Stars are nothing if not factories of chemical complexity.
12:44They slam particles together and create heavier elements, right, through a process called fusion.
12:50The later the generation of star, the more chemically complex it is.
12:54Toucana 2's spectral signature reveals its stars contain very few of these heavy, complex elements.
13:02A clue that lets astrophysicists calculate the age of the galaxy.
13:08These are very, very old stars from the very early days of the universe when the gas in the universe was not that chemically complex.
13:18Toucana 2 might be one of the oldest known structures that we can see in our local universe.
13:24It could be as old as 13 billion years.
13:27Almost as old as the universe itself.
13:31This grand old lady of a galaxy is a tiny thing, barely 3,000 stars.
13:39And yet, way out on her galactic rim, stars hurtle around at breakneck speed.
13:46When you look at the mass of this ultra-faint dwarf galaxy, it only has a few thousand times the mass of the sun.
13:53That's really small. And at the speed it's moving, it should fly apart.
13:57Toucana 2 doesn't break up because it's glued together, apparently, by an incredible amount of dark matter.
14:06When you look at a galaxy like our Milky Way, it's about 85% dark matter, which is a lot.
14:12But with Toucana 2, it's more like 99%.
14:16Toucana 2 is old, among the oldest galaxies in the universe.
14:21And it is packed full of dark matter.
14:24Simulations suggest this dark matter played a key role in shaping Toucana 2 and other very early galaxies right from the beginning.
14:34Gathering regular matter into clumps and building the first galaxies.
14:41The importance of dark matter really can't be overstated.
14:44It has actually controlled the way matter has evolved since the beginning of the universe.
14:48It brings matter together.
14:50You need this underlying structure of dark matter to make it all happen.
14:55Scientists think that for billions of years, as the early universe grew, dark matter called the shots.
15:03Without its gravity, structures like the Milky Way wouldn't have formed.
15:10We've seen dark matter's light-bending effects.
15:13We've even deduced where it should be.
15:16Dark matter really does appear to exist.
15:20But this evidence is indirect, circumstantial.
15:25To get conclusive proof that dark matter exists, don't we need to find some?
15:32If we could find a lump of dark matter, that would be one of the greatest discoveries in all of nature, in all of our history, right?
15:41Because we would understand one of the most fundamental components for how our universe works.
15:47Drop in the title. They love that.
15:50It's time to hunt for dark matter itself.
15:54Could it be hiding in the darkest place of all?
16:00Black holes.
16:12Scientists believe an invisible substance is pulling the strings in our universe.
16:19But until we see it, sense it, perhaps even touch it, dark matter is just a theory.
16:27Sometimes, though, ideas dreamed up by scientists come true.
16:33Like black holes.
16:35Once the stuff of science fiction and children's nightmares, black holes today are confirmed reality.
16:43So black holes and dark matter have a ton of similarities, right?
16:46You know, an unseen collection of matter that creates an enormous gravitational field, check.
16:51It bends light and causes gravitational lensing, check.
16:55Tests the boundaries of known physics, check.
16:58It seems crazy to even ask, but could our search for dark matter end in an idea more than 100 years old?
17:07Could dark matter be black holes?
17:12Black holes appear when stars explode, and their remaining mass crunches down into a sphere so dense even light can't escape its gravity.
17:22But that's where the black hole dark matter theory stumbles.
17:26We know that black holes happen. We know how they form.
17:29And we also know that there's nowhere near enough of them to be dark matter.
17:34Not enough stars have lived and died in the history of the universe to create 85% of the matter in it.
17:42If dark matter is made up of black holes, they would have to be an entirely different species.
17:48If dark matter is made up of black holes, they would have to be an entirely new type.
17:55It's possible that these black holes are of a type that we've never seen before.
18:00They could be primordial black holes.
18:05Primordial black holes are an idea, a theoretical concept at this point that we've never seen, but they could exist.
18:14If primordial black holes are real, then the universe is flooded with black holes.
18:20The smallest could have the mass of Mount Everest packed into the size of one atom.
18:25The biggest could be hundreds of thousands or millions of times the mass of the sun.
18:31Stephen Hawking first suggested that primordial black holes could be dark matter back in the 1970s.
18:40The idea centers on what happened during that intangible moment 13.8 billion years ago, the Big Bang.
18:50Theory says that primordial black holes formed in the first fraction of a second of the early universe.
18:57It's that time between when the universe goes from a pinprick to this giant inflating ball of gas.
19:04In these first moments of the universe's existence, matter is packed incredibly tightly, but it's not quite evenly spread.
19:14Even the tiniest fluctuations in density could trigger gravitational collapses.
19:21In other words, black holes would be forming everywhere, theoretically in huge numbers.
19:29By the time one second has passed in our universe, you're already making black holes thousands, hundreds of thousands of times more massive than our sun.
19:42The collective mass of these objects could be vast, but could they be 85% of the universe's matter?
19:51If primordial black holes really do exist, there might be enough to explain the dark matter.
19:58It's a tantalizing possibility, but there's one pretty big problem.
20:05For most scientists, the physics of the very early universe is incomplete and hard to trust.
20:12Generations of physicists dismissed primordial black holes as myths, fantasies, astrophysical unicorns.
20:21Until that is, an earth-shaking crash in space.
20:26May 2019.
20:29A violent cosmic event rocks the USA.
20:33How violent? Well, the physical distance between Louisiana and Washington State is stretched by nearly the width of an atom, which is bigger than it sounds.
20:44The Laser Interferometer Gravitational-Wave Observatory detects this wobble in space-time.
20:51This is the biggest gravitational wave event that LIGO has seen.
20:56This cosmic disturbance seems to come from colliding black holes, but crucially, not the ordinary dead star type.
21:06In this LIGO detection, one of the black holes is 85 solar masses. There's no way that a star could have made that black hole.
21:16Physicists believe there's a range of masses where dying stars can't collapse into black holes.
21:25Instead, stars in this zone become insanely hot and rip themselves apart, leaving nothing to crunch down into a black hole.
21:3685 solar masses sits right in the middle of this so-called forbidden mass range.
21:43The black hole that LIGO detected can't be a dead star, but in theory, it could be primordial.
21:51Could this discarded theory of dark matter be back in fashion?
21:56When the LIGO detections come up and everyone says, oh, right, primordial black holes, maybe we should pay more attention to that.
22:05Primordial black holes can be really appealing because they would solve the dark matter problem.
22:10But unfortunately, it's not that simple.
22:14The thing with flooding the universe with primordial black holes is that you expect a lot of collisions.
22:21And so LIGO shouldn't have seen one, it should have seen a thousand of these collisions, and we don't.
22:28Many scientists doubt what LIGO saw was a primordial black hole.
22:34To them, these beasts remain fairy tales of physics, red herrings in the quest for solid evidence of dark matter.
22:45Does dark matter exist, or are we chasing shadows?
22:51Some scientists think it's not only real, but that dark matter is within our grasp, and that it's flying through our bodies right now.
23:04We think 85% of the universe's matter is dark.
23:10And yet, we've never found a speck of it.
23:14We can't prove dark matter exists.
23:18Regular matter is made up of everyday particles, like electrons and protons.
23:24Scientists wonder if dark matter is also a type of particle.
23:30One of the leading candidates for dark matter are these things called weakly interacting massive particles.
23:37They're massive particles, like protons and electrons and things like that, but they don't interact well with normal matter, so they're weakly interacting.
23:44And they just have this name because it's awesome to call them weak.
23:47For decades, scientists have struggled to find these shy, theoretical particles.
23:53The very first physics research I ever did in my life was about actually measuring directly dark matter particles, these so-called WIMPs.
24:04And if they exist, then there would be a flux of millions of them in the universe.
24:11If dark matter is actually made of WIMPs, if these particles exist, then we're actually living basically in a sea of them.
24:19It surrounds and penetrates us, and it binds the galaxy together.
24:24WIMPs don't play by our rules.
24:28They barely interact with the world of regular matter.
24:31So they're hard to detect.
24:34But when they play with each other, sparks fly.
24:38Intense flashes that we just might be able to see.
24:43As the theory goes, WIMPs will self-annihilate.
24:47If WIMP-A and WIMP-B get too close together, poof, they explode and they create gases.
24:52Gamma rays are high-energy light, making them easy to spot.
24:59Scientists point their detectors at the center of the Milky Way, where they believe the WIMP collision rate should be especially high.
25:07We have a 4 million solar-mass black hole there.
25:10There are 4 million solar-mass black holes in the Milky Way.
25:13There are billions of stars there.
25:15That's where most of the mass of the galaxy is densest.
25:18So any WIMPs orbiting the galaxy will feel this natural attraction towards the center and fall toward it.
25:23The Fermi Large Area Telescope scoured the center of our galaxy for more than 10 years.
25:29It detected lots of gamma rays, but not all of them.
25:34The Fermi Large Area Telescope is the largest telescope in the world.
25:38It scoured the center of our galaxy for more than 10 years.
25:40It detected lots of gamma rays, but scientists couldn't tell if they came from colliding WIMPs.
25:48The galactic center is a mess.
25:50It's like downtown of a city, right?
25:52That's where everything is, where all the hustle and bustle is.
25:54There are stars exploding there, just tons of stars, gas, magnetic fields, a black hole, a lot of sources of gamma rays.
26:00So it's very difficult to tease out the signal.
26:03Downtown Milky Way was a washout.
26:05So the scientists turned their attention to planets living in less noisy zip codes, where WIMP collisions should be easier to spot.
26:14One place where you might see evidence for WIMP collisions is actually the cores of exoplanets.
26:20Turns out exoplanets might be the best dark matter detector we have.
26:26You can use giant planets orbiting distant stars as laboratories to understand dark matter.
26:33We know gravity should attract WIMPs.
26:36The more gravity, the more dark matter particles come together.
26:41Scientists suggest that WIMPs congregate inside the cores of the Milky Way's largest gas planets.
26:49In these supersized gas giants, WIMPs could collide, annihilate, and release gamma rays.
26:57If there are these WIMPs that are collecting in the centers of massive exoplanets, the annihilation of that dark matter can heat those exoplanets up.
27:04If you have a WIMP-heated exoplanet, and that's just fun to say, this thing is going to be warm.
27:10It's going to be warmer than the heat of space, which is very cold.
27:13So what you need is an infrared telescope, something that sees in infrared light and is sensitive enough to be able to measure the temperatures of these things.
27:21But a dedicated telescope like this won't launch until 2028.
27:27For some dark matter hunters, that's too long to wait.
27:32They argue that WIMPs do have one characteristic that should allow us to detect them right here on Earth.
27:40The key to detecting WIMPs is in their name. It's the W-I.
27:44They're weakly interacting. They're not not interacting. They do interact. It's just very weak with matter.
27:49And that means that there are the rare occasions where it will smack into a particle of normal matter, and then there are effects that we can observe.
27:58Scientists in Gran Sasso in central Italy watch for a spark of energy generated when a WIMP hits an atom of regular matter.
28:09Their detector, a tank of supercooled xenon, built thousands of feet beneath the Earth's surface.
28:17The beauty of putting this detector under a mountain is that you've got all of this rock and soil and everything else, which is blocking a lot of background noise.
28:25When you're looking for a WIMP interaction, you're looking for something that's very rare and something very subtle.
28:31So you don't want other things going on. You don't want other particles coming in and messing up your experiment.
28:36These weakly interacting massive particles will pass right through that mountain.
28:40And then if they smack into a xenon atom, we can look at it and go, ah, that was a dark matter particle.
28:45Detecting a WIMP could be definitive proof that dark matter exists.
28:51In 2020, the scientists spotted something in the results.
28:56But was it the elusive evidence, or a ghost among the stars?
29:14Scientists believe they can prove dark matter is real by detecting WIMPs.
29:21An experiment buried deep beneath an Italian mountain spotted unusual activity in a tank of regular matter, pure liquid xenon.
29:31So a WIMP detector, like the Xenon-1T, waits for a little WIMP, tiny, tiny little particle, to hit an atom of normal matter.
29:45And that creates a vibration. And we can see this entire block of xenon shake just a little bit from that little subatomic collision.
29:55The intensity of the vibration from the particle collision is critical.
29:59In theory, a WIMP striking a xenon atom should generate a powerful shock.
30:06The vibrations Xenon-1T detected were too weak.
30:12When a WIMP comes through, it smashes into the atom.
30:16It seemed like here something was just sort of rattling the electrons in the outside of the atom.
30:21So whatever is causing these detections was likely something much smaller than a WIMP.
30:26Let's take these results at face value.
30:29If they're correct, it's telling us that the dark matter isn't a WIMP, but something much, much smaller and something much, much lighter.
30:39The results suggest that what hit the xenon was actually a much smaller theoretical particle called an axion.
30:48Axions are really weird particles, incredibly light. In fact, almost zero mass is possible for an axion.
30:55An axion is no bigger than 150 billionth the size of an electron.
31:03Compared to a WIMP, an axion is like a soccer ball compared to our sun.
31:10The sheer tininess of axions makes them seem like an unlikely candidate for dark matter.
31:17If dark matter is real, it makes up 85% of the matter in the universe.
31:22To account for all that mass, we would need an almost unfathomable number of axions.
31:30140 trigontillion of them, in fact. That's 140 with 93 zeros after it.
31:38If axions exist, space must be swimming with them.
31:44They must be packed into every corner of the cosmos.
31:47When regular matter clumps together, it forms stars.
31:51So, to prove dark matter exists, maybe we should be looking for dark stars.
31:58There's no reason they can't exist. There's even a name for them.
32:03Ghost stars.
32:06They're very weird objects. These ghost stars are like nothing we would ever see in the night sky.
32:11We've never seen a ghost star. They are theoretical objects made of hypothetical axions.
32:18But, in theory, ghost stars should form like any other star, pulled together by gravity.
32:26They would be gigantic, super-dense objects floating through space.
32:32They could reach the mass of tens of millions of suns.
32:36But because they are made of dark matter, ghost stars would produce no energy and emit no light.
32:43They would be transparent to both light and matter.
32:48If you were right next to it, you wouldn't even notice it, right?
32:52If we sent a probe through it, it'd sail right through it.
32:55And once it passed through, it'd be pulled back by its gravity.
32:5985% of the mass of the universe is dark matter.
33:0485% of the matter in our universe could consist of transparent orbs made of infinitesimally small dark matter particles.
33:16But do these invisible stars exist?
33:20The evidence is thin, but...
33:23Rewind back to that LIGO detection in 2019.
33:27The gravitational wave detector picked up the signal of two massive objects colliding.
33:36We call the event GW-190521.
33:41Most scientists agree this was a black hole collision.
33:45But could it have been clashing ghost stars?
33:49If there are ghost stars out there, and they can interact with each other gravitationally,
33:53they may collide. And when they do, they would emit gravitational waves.
33:58And it would look a lot like two black holes colliding.
34:02In fact, it would look, theoretically, very much like GW-190521.
34:07One collision, two explanations.
34:11Primordial black holes, or ghost stars.
34:16LIGO can't tell them apart.
34:19Do these ideas bring us closer to proving the existence of dark matter?
34:24Or are we just hurtling further down a weird physics rabbit hole?
34:30Primordial black holes, ghost stars, axions.
34:35This is all very exotic physics.
34:38We can't take for granted that any of this is real, or that it's not real.
34:42We just don't know.
34:44Dark matter is irritating.
34:46Oh, we know it's out there. We see its effects, right?
34:50But we can't see the dark matter.
34:53And that's frustrating.
34:55And it's like a lot of young fields in astronomy.
34:59We have way more ideas than we do hard observations.
35:04We have ideas. We have theories.
35:07But without direct observations, we just can't back them up with solid proof.
35:11The more we look, the harder it is to find dark matter.
35:17Maybe it's primordial black holes from the early universe.
35:22Maybe it's a sea of particles that flow right through us every day.
35:28Or maybe it's gigantic, transparent ghost stars.
35:33Perhaps it's the combined mass of Santa's sleigh and the Easter Bunny's belly.
35:37Perhaps it's the combined mass of Santa's sleigh and the Easter Bunny's basket.
35:43Or maybe all our physics is based on questionable math.
36:0085% of the stuff in the universe is missing in action.
36:04The search for this dark matter looks hopeless.
36:09This problem of dark matter is really a tough one.
36:13Everything that we've predicted and then gone and looked for, we're not finding it.
36:17It's starting to become a huge embarrassment.
36:20Surely something so fundamental to our cosmology should be detectable.
36:25And yet, it remains elusive.
36:28We're stumbling blindly around the limits of our understanding.
36:32As of right now, there are zero direct observations.
36:37Maybe dark matter doesn't exist after all.
36:41Instead of searching for an invisible substance affecting the universe with its gravity,
36:47maybe it's gravity we don't quite understand.
36:52If you're looking at a galaxy and it's spinning way too quickly,
36:56either there's a new ingredient in the galaxy like dark matter that holds it all together,
37:03or you're misunderstanding the laws of physics.
37:07To describe the effects of gravity, we use the nearly 350-year-old math of Sir Isaac Newton.
37:16Maybe to explain the excess gravity we see in the universe, it's not extra matter we need, it's better math.
37:25Although we understand very well how gravity works here on Earth and in our solar system,
37:30perhaps when you get up to galactic scales, it actually behaves just slightly differently.
37:36And if that were the case, you can kind of tweak that idea until it fits the data we see of how galaxies are spinning around without needing dark matter.
37:46Questioning the math of a legend of physics might sound like sacrilege.
37:50But to solve the dark matter conundrum, it has been done.
37:55It's called Modified Newtonian Dynamics, or MOND.
38:00Modeling galaxies with this math produces very different results.
38:07On its surface, MOND is not a bad idea.
38:11In the same way that we would normally program a computer to include dark matter in our simulations,
38:17you can take that out and instead program it with a different law of gravity, with MOND.
38:23And then you can set up a kind of spinning mass of gas.
38:27And it does seem to be possible with MOND to get things that settle down and look a bit like a real galaxy.
38:35Changing the law of gravity accurately recreates the super-fast spin astronomers see through their telescopes.
38:43No need for dark matter. It doesn't exist.
38:48Case closed? Not by a long shot.
38:51With anything bigger than a galaxy, this artificial physics breaks down.
38:57MOND does really well on galaxy scales.
39:01But when you zoom out and you go to larger and larger structures on our universe, like clusters of galaxies and big, big structure,
39:08you see that MOND by itself can't reproduce all our observations. There's something missing.
39:15Dark matter. Dark matter, dark matter, dark matter.
39:18Dark matter. In MOND you still have to invoke the existence of material you can't see.
39:24It basically introduces some of its own dark matter as well, which kind of negates the point of having MOND in the first place.
39:34MOND doesn't replace dark matter.
39:38The universe still needs something to hold it together.
39:42We just don't know what it is. But there are plenty of new ideas flying around.
39:48In my theory, the dark matter is a superfluid.
39:52It's a radical new theory of dark matter.
39:56Particles, not acting individually, but flowing as one invisible mass around the galaxies.
40:05A superfluid is like an ordinary fluid that flows, but in this case it flows without any resistance or viscosity.
40:12If I pour honey, it will flow very slowly. It has high viscosity.
40:17A superfluid will just flow and never stop flowing.
40:21As the superfluid dark matter flows around the universe, eddies and waves form, large enough to engulf entire galaxies.
40:31The gravity of the fluid holds the stars together.
40:36But, like most theories on dark matter, there's no direct evidence.
40:42If these waves are on the size of galaxies, then we have to find detectors that can detect those types of huge waves.
40:50They don't exist at the moment.
40:53Which brings us back to square one.
40:56We just can't prove that dark matter is real.
40:58Primordial black holes, ghost stars, wimps, a superfluid sloshing about the cosmos.
41:06Or, maybe we're just using the wrong math.
41:10What's your money on?
41:13If I had to wager $20 on what dark matter is?
41:18Hmm.
41:20I would never place money on what dark matter is. I just think we have no idea.
41:24My money is on dark matter itself is real, but it's not the whole picture.
41:32I would say, left socks and dryers.
41:36I would say, remote controls that fall into sofa cushions and disappear.
41:40I would love there to be dark matter ghost stars, planets, even dark matter people.
41:46I'm going all black. I think no current ideas are correct.
41:52I think dark matter is something that we haven't thought of yet.
41:58Does dark matter exist?
42:01Watch this space.