[MUSIC PLAYING] Dark matter literally binds the galaxy together.

But there's a problem.

Nobody knows what dark matter is.

My name is Matt and this is "SpaceTime."

Physics has a problem.

The Milky Way galaxy is spinning so fast that it should be scattering its stars into the void.

Based on the amount of binding gravity that we calculate from everything we can see, we can only account for 10% of the mass needed to hold its stars in orbit.

So what's wrong?

Either we're missing and frankly don't understand at least 80% of all the matter in the universe or our current understanding of gravity is wrong.

This is the mystery of dark matter.

Now, before we get into figuring out exactly what dark matter is or isn't, I want to give you completely independent evidence for its existence, gravitational lensing.

Thanks to general relativity, we know that light fall is the curved geodesics of a gravitational field.

Place a strong gravitational field on an axis between a light source and an observer and voila, you basically have a lens.

And galaxy clusters do this all the time, turning the background universe into a funhouse mirror of stretched out and duplicated galaxies.

From this, we can figure out exactly how much mass is needed to cause the observed lensing.

But again, we find the clusters appear to have way more mass than we see in the stars alone, that is if we understand gravity.

So knowing this, let's summarize the actual possibilities for dark matter.

One, best case scenario, it comes from particles that we've already discovered, just in a form that's very difficult to detect.

Two, not so great, dark matter is a type of particle that's beyond our current understanding of particle physics.

Or three, even worse, we're actually not missing any mass.

Gravity just behaves differently on the vast scales of galaxies and clusters.

So general relativity, wrong.

OK, let's start with the first possibility.

The standard model of particle physics is basically the periodic table of known fundamental particles and fields.

It underpins everything we know about the subatomic universe.

If dark matter exists in this model, its mass probably needs to come from protons and neutrons.

But they can't be interacting with light.

If this is dark matter, the galaxy would need to be swarming with baryonic things as massive as stars, but that are so compacted that they're basically invisible.

Is this even possible?

Actually, it is.

They're called MACHOs, massive compact halo objects.

And they're basically crunched down, compact, dead or failed stars, black holes, neutron stars, brown dwarfs, Macaulay Culkin, et cetera.

And they are very hard to see.

But we can see these guys, at least sort of, with gravitational lensing.

The alignment has to be perfect.

But when, say, a black hole passes between us and a more distant star, we sometimes see a brightening of that star.

Astronomers spent years counting MACHOs this way.

And they found plenty.

But not nearly enough to account for all of the dark matter.

So option one is out, which means we're left with two bad choices.

Either particle physics is wrong, or at least horribly incomplete, in that we're missing 80% to 90% of the mass in the universe, or Einstein is wrong.

Sacrilege, right?

Remember when I said that the Milky Way spinning too fast?

Well, the problem is that the stars on the edge of the galaxy are moving just as fast as the stars near the center.

But they should be moving slower because the gravity out there should be weaker.

According to Newton, gravity weakens proportional to distance from its source squared.

This relationship is definitely true on the scale of the solar system.

But what about the entire galaxy?

Could it be that what we see as dark matter just comes from gravity behaving differently on truly gigantic scales?

Well, it turns out that if you make a simple change to Newton's gravity, things work out.

Give gravity a bit more staying power, make it drop off proportional to distance instead of distance squared, and then you don't even need dark matter.

The stars alone give you plenty of gravity.

The original modified Newtonian dynamics hypothesis, and it's a general relativity extensions, tries to give us this basic relationship for gravity.

1 over R squared at small scales, 1 over R at large.

But you can't just break general relativity and start over.

Any replacement theory has to reproduce all, and I mean all, of the verified predictions of Einstein's theory and be able to explain dark matter.

Modified versions of GR can actually do pretty well, especially predicting orbits within galaxies.

But they ultimately have a hard time getting all of the observed effects.

They either need some serious fine-tuning or you have to add back in some actual dark matter particles, which kind of defeats the purpose.

But there's an even bigger nail in the coffin of modified gravity.

Say hello to the Bullet Cluster.

It's actually two clusters that smashed right through each other.

The gas was ripped away from the stars and now lives between the clusters.

In the Bullet Cluster, most of the mass actually is in the gas.

So if dark matter really comes from weirdly behaving gravity, then the cluster's gravity should stay concentrated on the gas.

But if dark matter is an unseen particle, and it's the type of particle we think it might be, then that dark matter should pass right on through, just like the stars.

How do we test this?

Again, gravitational lensing.

Map the mass based on the warping of light from more distant galaxies.

And we see that in the Bullet Cluster, the dark matter is with the stars.

This tells us that matter is a real particle, not just broken gravity.

Once again, Einstein prevails.

Dark matter exists and it represents, if not broken, at least incomplete particle physics.

But what do we know about it?

Well, it's slow and it's heavy.

And those two go together.

It has to be pretty slow moving, or cold, because we know that dark matter clumps together gravitationally to build galaxies and clusters.

Remember the hot, smooth plasma way back in the early universe that produced the CMB?

And the last guy talks about it here.

Well, in order to go from that highly smooth ocean of orange plasma to today's highly structured universe of clusters and galaxies, something had to act with enough gravity to pull stuff together.

There's no way there's enough regular matter to do that.

Dark matter, as well as binding the galaxy together, is also the main force in forming galaxies in the first place.

No dark matter, no galaxies.

And even then, galaxies could only have formed if dark matter particles are cold, massive, and weakly interacting.

Weakly interacting massive particles, WIMPs, actually refers to a specific and popular contender for dark matter.

WIMPs are a family of particles that may arise out of supersymmetry.

This is a funky extension to the standard model of particle physics.

Now, there's a lot to supersymmetry.

But, in short, versions of this theory predict the existence of a set of counterparts to the familiar standard model particles, but that are hundreds of times more massive.

Some of them fit the bill for dark matter.

Sinking down into the depths of quantum field and string theory, you can find all sorts of strange fish, WIMPs, axions, neutralinos.

Some of which may actually exist and some of them may be dark matter.

But it's all mathematical fantasy until we detect the particle.

We have detectors here on Earth designed to catch the fall-out between the unthinkably rare collisions between a dark matter particle and an atomic nucleus.

We also watch the heavens for the equally elusive gamma radiation produced when dark matter particles annihilate each other out in space.

There's a big fat Nobel Prize waiting for the scientists who figure this one out.