Dark Matter: Cool name, but actually what is it?
Dark matter is the name of some weird stuff in the Universe, that we know exists, but don’t really understand. We know it exists because we can see it pulling on galaxies and stars using gravity. In fact, looking across the whole Universe, we know there is a lot of it, more than there is of the normal matter we see around us on Earth. It tends to clump around at the center of galaxies and in dense regions of the Universe. But it doesn’t seem to interact with light, so we can’t see it using normal telescopes. In fact, it doesn’t really seem to interact with anything except gravity. Let’s dive in, to dark matter (Image: Chandra X-Ray Observatory NASA).
What is Dark Matter? How did we find it?
Some stuff in science we understand really well. We know the Earth is about 5 billion years old, we know about the Big Bang and evolution. However, there is other stuff we really don’t understand. And a lot of that stuff, has really silly names. “Dark Matter, “Dark Energy”, we know they exist, but know nothing about them.
Dark Matter was discovered by accident. It is pretty easy to work out how heavy a galaxy is. You point a telescope at the sky and count the number of stars and get an estimate of how big they are. You then add up all this mass, and voila, you get a pretty reliable estimate for the mass of the galaxy.
However, the problem is, this number cannot be right. If you actually look at how fast galaxies are pulling stars towards them and how fast they are rotating, most galaxies act as if they are much heavier than the weight of all the stars and visible matter in them combined.
The fact that galaxies seem to have much more mass than the total weight of the combined stars, leads to a pretty clear conclusion. There is a bunch of “dark” heavy stuff in a lot of galaxies that we can’t see.
Now, this does seem weird, so a lot of scientists have double-checked these measurements and tried to come up with other explanations. But over about 50 years of research, with increasingly precise measurements of the mass of different galaxies, depending on how they are moving and spinning, it has become really clear. There is a big clump of dark stuff at the center of almost all the galaxies. And we can’t see it with normal telescopes.
Okay, so what is “Dark Matter” then?
The short answer is, we don’t know what dark matter is. Dark matter doesn’t interact with light. That’s basically a fancy physics way of saying that it is completely transparent – it doesn’t block out light from the stars behind it. So a lot of the standard tools we have for studying things out in space, like looking at their color or shape, just don’t work.
The fact dark matter is transparent means that dark matter is probably not made of atoms. Atoms almost always block out light. Instead, it’s probably some completely new type of stuff formed from different types of particles. Lots of people have theories about what sort of particles it could be, but we really don’t know. We are doing lots of experiments in particle colliders trying to make some dark matter, but we have not had a lot of luck.
In theory, if dark matter is made of some other type of particles, billions of these should be passing through the Earth every second. To try and detect these particles, we have built laboratories deep within the earth, miles underground to shield it from other radiation which might mask the signal, such as light or cosmic rays. The results of these experiments are only just starting to come in, but so far, we have not been able to detect any signal of dark matter. It just seems to be completely invisible. It’s really weird.
Now, this is a real problem in physics. Because dark matter isn’t rare. In fact, if we measure the mass of all the galaxies, there is more dark matter in the Universe than real matter. Not on earth or in our solar system, that’s all normal matter. But if you point a telescope at the sky and look at a whole galaxy, about 80% of the mass of that galaxy is this dark matter.
It is also likely that dark matter has had some fundamental effects on the evolution of our Universe and the Big Bang (see our posts on all that here). This again is something we still don’t understand.
Some people like to say that Science ruins the magic of the Universe. These people didn’t pay attention in science class. There is still so much weird and mysterious stuff going on in the Universe. We have so much more to learn. We, at The Universal Story, are so excited to find out more.
