WOMAN: Here's a fun question.

What do you do with the LHC, a $50 billion science experiment, once it achieves its primary goal?

Well, you tackle the biggest unanswered mystery in physics, a mystery that began in the 1960s when astronomer Vera Rubin looked up at the Andromeda Galaxy and noticed something puzzling.

The stars at the edge of the galaxy were moving much too fast, spinning around up to 10 times faster than our theories predicted.

The only way this was possible was if something's there we could not see-- dark matter.

For decades, scientists have looked for this stuff here on earth with detectors deep underground, detectors filled with liquid xenon, detectors with crystals.

And they've looked and looked and found nothing.

So how are we so sure dark matter exists?

What is dark matter?

And what does this all have to do with the most advanced scientific experiment in the world-- the LHC?

Hey, I'm Dianna, and you're watching Physics Girl.

I recently visited CERN in Geneva, Switzerland, which is home to the LHC.

I spoke with a theoretical physicist about what the LHC has to do with dark matter.

So, I'm Dorota Grabowska.

I'm a CERN senior fellow.

So that's sort of a postdoc at CERN.

I work on sort of two different subject matters.

So one is dark matter.

The other one, which is sort of completely unrelated, is more formal work, so trying to actually understand quantum field theory.

DIANNA: My first question was, What do we currently think dark matter is?

We actually have very little understanding about the mass of dark matter.

The mass range that dark matter can cover is 90 orders of magnitude.

Our knowledge about the dark matter mass is equivalent to asking, Is my dark matter object the mass of a neutrino or the entire visible universe?

That's 90 orders of magnitude.

We will get to the wild possibilities of what dark matter could be.

But first we have to answer-- If it's been so hard to find, and if we can't even see it, how do we even know dark matter is there?

Good question, Dianna.

We can see the biggest piece of evidence for dark matter right here in our own Milky Way galaxy.

Just like Andromeda, the Milky Way could not exist, it could not spin as fast as it does, without dark matter.

But unless you're an astronomer who already had expectations on how galaxies are supposed to spin, it's a little hard to relate.

So let's try this.

You know that thing where you hold hands and spin around each other?

The faster you spin, the harder you have to hold on or you'll fly apart.

Same applies to galaxies.

The faster the stars spin around the galaxy, the harder something has to hold onto them to keep them from flying off into space.

And the only thing that can hold them is gravity.

So you need mass.

The more mass a galaxy has, the faster it can spin.

Smaller galaxy, it has to spin slower.

Now here's where it gets weird.

We look up at galaxies like the Milky Way and we look at all the matter we can see.

We do some calculations, some math to figure out how fast stars can be moving around the galaxy.

And we find that galaxies are spinning way faster than they should be able to, according to our laws of physics.

So we know something else is there in the galaxy, something that we cannot see, and a lot of it.

Some estimates put the makeup of dark matter in the Milky Way galaxy at 90% to 95%.

At this point, I often get the question, Is it possible that we just don't understand gravity?

Like maybe on huge galactic scales, gravity just acts differently.

Not a bad question.

In fact, this looks so good, some scientists have worked on a modified theory of gravity, some messed-up gravity math.

With the fact that we see dark matter effects not only in galaxy rotation curves but also this cosmic microwave background with baryon acoustic oscillations, we're pretty sure that it has to be some type of matter that we just haven't discovered yet.

So that's it.

We have other evidence that points to dark matter being stuff.

For example, we see galaxies in the Coma Cluster moving around each other too fast.

In fact, that was the first evidence for dark matter, observed by astronomer Fritz Zwicky in the 1930s.

Then there are instances like the Bullet Cluster, where two galaxies ran into each other.

And some of the mass collided and stayed there, like you'd expect in a crash.

And some mass passed right through like a ghost and didn't interact.

And the weirdest evidence for dark matter is that some galaxies don't have it at all.

Seems kind of counterintuitive.

But if dark matter was a quirk of the math, then you would expect to see it everywhere.

And you don't.

We have other evidence for dark matter pointing to it actually being stuff.

In fact, there's so much evidence that we even have an estimate for how much of the universe is made of dark matter.

There's this graph that you'll often see with physicists talking about dark matter, and it's a little confusing because physicists like to clump energy and matter together in the same graph.

So that's what this graph does.

When you look at all of the modern energy in the universe, 26.8% of it is made up of dark matter.

That leaves 73.2.

Is that right?

Hey, Siri?

SIRI: Plus 26.8 is 100.

DIANNA: Is the rest of it normal matter?

No.

68.3% of it is dark energy.

Dark energy is a whole other can of worms, and we're not going to get into it in this video.

OK, but then that only leaves 4.9%.

All of the normal matter that we know of-- protons, neutrons, and even the weird stuff like neutrinos and quarks, strange quarks, and charm quarks-- all of that stuff only makes up 4.9% of the matter and energy in the universe.

Chew on that for a minute.

So if there's so much dark matter in the universe, why has it been so difficult to find?

Well, with other discoveries, like stars, we saw them.

With quasars and galaxies and Pluto, we saw them.

Dark matter, nope, can't see it.

What about with black holes?

We knew what they were first, and then we saw them.

With dark matter, you see the issue?

DIANNA: So you say-- you say a different classification.

And then like, you work on this type of dark matter.

Does that mean, like, that we know there are these types of dark matter?

Or they're still all theoretical?

They're still all theoretical.

So we don't know what dark matter is yet.

But we have candidates at very different mass ranges.

I would go through my sort of mass regime.

Primordial black holes are at the very top.

There's questions of whether you could produce black holes in other ways.

So not sort of from inflation, or not stars, but actually having dark matter creating sort of these like dark nuclei or dark neutron stars that then undergo their own gravitational collapse.

Why not try it?

I've been trying to figure out a way to do it for years.

And so far, I have not come up with a way that'll actually do it.

Hold on.

You've been trying to figure out a way to do it.

That sounds kind of funny, 'cause it sounds like you're trying to make dark matter black holes, which would be awesome.

But you mean trying to figure out a way mathematically that that could happen.

Yeah, so doing a model, right.

So essentially, if I think about these primordial black holes in the typical way, people write down a model of inflation.

They can figure out how space-time will look during inflation, say, "Ah, given this model, "I can see, you know, that these sort of "deep gravitational potentials can form.

That'll be these primordial black holes."

Black holes formed at the beginning of the universe are one of the candidates for dark matter.

So those are usually solar mass.

That's sort of the typical heavy dark matter candidate.

There is a massive jump going from primordial black holes to WIMP and asymmetric dark matter.

And then going down further, you get things like axioms or axiom-like particles, very weakly interacting, bozonic, sort of scalar fields.

And as for WIMPs, the other particle she mentioned, which is probably the most commonly searched-for class of dark matter particle candidates, because they're predicted by supersymmetry.

But no one has found evidence for supersymmetry, nor has anyone found any WIMPs, not even the lab I worked with that was searching for dark matter when I studied physics.

I work on a type of dark matter that actually lives between, like, the Planck scale and the primordial black hole scale, where it's essentially a composite dark matter.

So it can be heavier than a Planck mass.

So dark matter could be none of the above or all of the above.

It doesn't have to be any one thing.

It could be 40% birds, 60% cake batter, metaphorically speaking.

Back to our original question.

Is the LHC our best chance of finding dark matter?

It kind of seems like an unrelated area of physics.

The LHC just smashes protons together.

I mean, not just.

Like it smashes them together at a higher energy than any other lab in the world.

And when it smashes them together, a shower of very strange debris comes out.

GRABOWSKA: So one of the things that ATLAS and CMS look for is essentially when they collide protons, depending on the coupling to the dark sector, and exactly the mass of the dark matter particle, they can actually produce it in collisions.

They could theoretically produce it in collisions.

So there's the idea of directly creating dark matter at the LHC and then detecting it at one of the four main detectors along the ring.

And what would our results depend on?

So there are constraints on certain types of dark matter, that the two groups, the two collaborations actually put constraints on.

So every time you have a dark matter candidate that's sort of in the specific mass range, you go ask CMS or ATLAS.

You look at their papers on dark matter constraints to see whether you're already ruled out or not.

So depending on what physics we've already seen with collisions in ATLAS and CMS, theories can rule out dark matter particles of specific mass ranges.

So there are a lot of creative ways to look for dark matter with the LHC in a way that no other lab on earth can do, because only the LHC can produce collisions this explosive.

Do I think that we're any closer to finding dark matter because of the LHC?

I don't know.

I honestly don't know.

I'm still hung up on the fact that we can only see 5% of our universe.

And we have no idea what the rest of it is.

But why not turn all the incredible resources we have towards finding it?

I'm excited to see, when the LHC turns back on, what kinds of results we get in the regime of dark matter searches.

And in Vera Rubin's own words, "I wonder if the explanation is even more complex than we imagine at present."

Thank you guys so much for watching this video.

Stay tuned for the next video, which is going to be our final episode on CERN.

But in the meantime, happy physics-ing.

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