What is the Dark Matter? The mystery of the Universe
Dark matter. We only call it dark matter because it doesn’t interact with light. Yep. 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. That’s a pretty physics-y statement, not quite as sci-fi. But, 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. 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, an astronomer named Fritz Zwicky was studying a cluster of galaxies called the Coma Cluster. What’s the force holding orbiting galaxies together? 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. Well, no problem. 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. And 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. This was the first hint that there might be something out there he couldn’t see. He called it dark matter. 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. The gravity from giant objects bends light. It’s almost as if there were a giant lens in between you and the star. 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. They’re moving around the galaxy faster than they should be. Many people have speculated dark matter is just a bunch of MACHOs. 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 MACHOs. 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 WIMP. A WIMP is a Weakly Interacting Massive Particle. I’m not just going to tell you what it is. What do you think WIMPs interact weakly with? And why do you think they might have to be massive? These WIMPs 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 WIMP, 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.