What is Dark Matter? | Can it be created?

What is Dark Matter?

Dark matter, sounds pretty dramatic, huh? Like some sinister goo under control of the Sith Lords. Maybe that's what dark matter is. Although you might not think so if I told you we only call it dark matter because it doesn't interact with light. It doesn't reflect light, doesn't absorb light. It doesn't interact with electromagnetic radiation, or light at all. That's why we call it dark. I know, that's a pretty physics statement, not quite as sci-fi. But I promise you, there are still so many mysteries surrounding dark matter. And we're going to talk about what dark matter might be, how we know it's there, how we know what it's not, and why we might be wrong about all of this, in the end.

So dark energy makes up 69% of the energy density in the universe and 26% is dark matter and about 5% is baryonic matter, or ordinary matter. This 5%, the baryonic matter, that includes me, you, our Earth, the solar system, all of the stars, the Milky Way galaxy, all of the galaxies-- everything we can see. It's all only 5% of the energy in the universe. So then how do we know that? And how do we know that 26% of the rest of the stuff in the universe is dark matter? That's the first question I had. If we can't see dark matter because it doesn't interact with light, so how would we see it, since we only see things light-- how do we know it's there in the first place? Well, how do we ever know something's there that we can't see? What about wind? You can't see the air moving, but you can see its effects on other things. Such was the case with dark matter.

All the way back in 1933, this dude named Fritz Zwicky was studying a cluster of galaxies called the Coma Cluster. See, galaxies are like mean girls and are often found in clusters. When galaxies orbit one another, they're often playing this game. The faster they're going, the stronger they need to hold on to one another and what's the force holding orbiting galaxies together? It's Gravity. The faster they go, the more gravitational attraction they need to hold them together. And we know more gravity only comes from adding more mass. You look at how fast the galaxies are orbiting, and you calculate how much mass is needed for the gravitational force to hold them together. You find that there is mass missing. Zwicky saw the galaxies moving faster than the gravity of the mass. He could see,  could hold them together. Dianna Cowern was the first hint that there might be something out there he couldn't see. He called it dark matter. If you're thinking, that's it? I could come up with a million ways to explain what Zwicky saw.

Well, that wasn't the end. The next evidence for dark matter comes from a totally different but crazy phenomenon.  When you're looking at a star, the light from the star is usually coming straight at you. But when you place a giant object, like a galaxy, or a cluster of galaxies, in between you, then the light going this way is bent around. Yeah, the gravity from giant objects bends light. It's almost as if there were a giant lens in between you and the star. Shockingly, we call this gravitational lensing. So depending on how much the light is bending, we can estimate the size of the galaxy. Because again, more mass, more gravitational pull on the light. So then we look at the galaxy and we calculate all the matter we can see, and they don't add up. There's missing mass, again! What do you think it might be?

Now, there's even more evidence for dark matter. And it has to do with how stars are moving around the galaxy.  Many people have speculated dark matter is just a bunch of Massive Compact Halo Objects. These are things like brown dwarfs, white dwarfs, even black holes-- things that have very low luminosity, or light emanating from them, but are small and dense. Well, some of the missing mass might be these Massive Compact Halo Objects. But cosmological models of the early universe have shown that we shouldn't see more than this 5% level of baryonic matter which happens to be the exact number that we measure. So then couldn't dark matter just be a bunch of dark dust and gas? Well, same argument. That's all baryonic matter, as well. It can't account for all the missing matter. So again-- what is dark matter?

Well, the leading theory is dark matter is likely a Weakly Interacting Massive Particle. I'm not just going to tell you what it is. I'm going to leave you with a few questions. What do you think Weakly Interacting Massive Particles interact weakly with? And why do you think they might have to be massive? These Weakly Interacting Massive Particles couldn't interact with the electromagnetic force, or light, as far as we can tell. So scientists have set up detectors deep underground, hoping that a dark matter particle, a Weakly Interacting Massive Particles, will bump into a particle in the detector, although infrequently, and will get a signal.

If dark matter doesn't interact with regular matter at all, besides through the gravitational pull that we've seen, then we have no chance of directly detecting it. For now, we can only hope it does. Well, I hope this gives you a sense of what dark matter might be, what it might not be, what it is, what it's not. But remember, science is an ever-growing and changing field of knowledge. And because dark matter is so new and unknown, we might yet prove ourselves wrong on all of these hypotheses. But for now, we're left with the evidence. And it points to dark matter!

Can Dark Matter be created?

85% of the matter in our universe is a mystery. We don't know what it's made of, which is why we call it dark matter. But we know it's out there because we can observe its gravitational attraction on galaxies and other celestial objects. We've yet to directly observe dark matter, but scientists theorize that we may actually be able to create it in the most powerful particle collider in the world. That's the 27 kilometer-long Large Hadron Collider, or LHC, in Geneva, Switzerland. So how would that work? In the LHC, two proton beams move in opposite directions and are accelerated to near the speed of light. At four collision points, the beams cross and protons smash into each other. Protons are made of much smaller components called quarks and gluons.

In most ordinary collisions, the two protons pass through each other without any significant outcome. However, in about one in a million collisions, two components hit each other so violently, that most of the collision energy is set free producing thousands of new particles. It's only in these collisions that very massive particles, like the theorized dark matter, can be produced. The collision points are surrounded by detectors containing about 100 million sensors. Like huge three-dimensional cameras, they gather information on those new particles, including their trajectory, electrical charge, and energy. Once processed, the computers can depict a collision as an image. Each line is the path of a different particle,
and different types of particles are colour-coded. Data from the detectors allows scientists to determine what each of these particles is, things like photons and electrons.

Now, the detectors take snapshots of about a billion of these collisions per second to find signs of extremely rare massive particles. To add to the difficulty, the particles we're looking for may be unstable and decay into more familiar particles before reaching the sensors. Take, for example, the Higgs boson, a long-theorized particle that wasn't observed until 2012. The odds of a given collision producing a Higgs boson are about one in 10 billion, and it only lasts for a tiny fraction of a second before decaying. But scientists developed theoretical models to tell them 6what to look for. For the Higgs, they thought it would sometimes decay into two photons. So they first examined only the high-energy events that included two photons. But there's a problem here. There are innumerable particle interactions that can produce two random photons.

So how do you separate out the Higgs from everything else? The answer is mass. The information gathered by the detectors allows the scientists to go a step back and determine the mass of whatever it was that produced two photons. They put that mass value into a graph and then repeat the process for all events with two photons. The vast majority of these events are just random photon observations,
what scientists call background events. But when a Higgs boson is produced and decays into two photons, the mass always comes out to be the same. Therefore, the tell-tale sign of the Higgs boson would be a little bump sitting on top of the background. It takes billions of observations before a bump like this can appear, and it's only considered a meaningful result if that bump becomes significantly higher than the background.

In the case of the Higgs boson, the scientists at the LHC announced their groundbreaking result when there was only a one in 3 million chance this bump could have appeared by a statistical fluke. So back to the dark matter. If the LHC's proton beams have enough energy to produce it, that's probably an even rarer occurrence than the Higgs boson. So it takes quadrillions of collisions combined with theoretical models to even start to look. That's what the LHC is currently doing. By generating a mountain of data, we're hoping to find more tiny bumps in graphs that will provide evidence for yet unknown particles, like dark matter. Or maybe what we'll find won't be dark matter, but something else that would reshape our understanding of how the universe works entirely. That's part of the fun at this point. We have no idea what we're going to find. 

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