What is Dark Matter?

Question posed by Jen.

I, ah... I ... don't know.

That's not really an admission of my fallibility, though*: no-one knows. 'Dark Matter' is a name that scientists have given to something that we don't know much about, except that it must be there.

How do we know it's there?

Matter bends space. If you get enough matter in one place, it bends space a lot, and you can start to see the effect that this has - it's what makes moons orbit around planets, and planets orbit around stars,and we call it gravity.

Thanks to very clever people like Sir Isaac Newton, we know rather a lot about how gravity shapes the universe, and can make some pretty accurate predictions about what might happen in different situations: we've sent spacecraft to various objects in our solar system, and have managed to not miss most of them.

Inside our solar system, everything works as our theories say it should. But when we look outside at other galaxies, we find that thinks don't quite pan out as we think they're going to. You can read a more in-depth discussion of that in this post, but the short version is that as you look further away from the centre of galaxies, things act more and more like there's more matter there than we can actually see.

It isn't even a small difference, which would allow us to think that there's just a bit of stuff hiding behind some other stuff, and we'll see it at some point when things have moved around a bit: it's a huge, enormous, whacking great discrepancy. This is how big the issue is:

Around 83% of the universe appears not to be there.

Put another way, the matter that we can see swirling around in galaxies accounts for only 17% of what our best theories say should be there, based on how it's all moving around.

What do we know about Dark Matter?

We're looking for a type of matter that has the following characteristics:

  • It has mass (so it's matter).
  • It does not emit, reflect or scatter light or any other electromagnetic radiation (so it's dark).
So it's called Dark Matter because that is, literally, all we know about it: It's some kind of matter that is dark.

* There'll be a lot more kicking and screaming before that happens...


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