Question: what is the theory of dark matter? what is it actually about?

Miriam Hogg answered on 8 Nov 2018:

This is a great question.

We have the universe we ‘see’, all the stars, planets, black holes. That is the visible matter makes up only 5% of the universe. The way we realised that there was more than we could see was by looking at galaxies. Using all the physics and maths we know we found that stars in their galaxies should orbit (move around its central black hole) at a certain speed (based on how far away it was from the centre). So they looked at other galaxies and measured the speed of the stars. Some of them matched their predictions. But some were moving much faster! The only way they could be moving faster was if there was more mass than we could see. This is what we call dark matter and that makes up 25% of the universe.

Some galaxies have lots of this dark matter that affects the speed of the stars orbiting, and some have very little. But we don’t know what it is made of! It could be lots of little black holes that we cant see but make the galaxy more massive than we think, or it could be a big particle that doesn’t interactive with light but does with mass (so we can’t see it but it’s still there). It’s a difficult idea because we know something is there but we can’t see it.

The other 70% of the universe is made up of what we call ‘Dark Energy’ which is a force that pushes matter apart, (gravity pulls things together so this is kind of like an opposite force). We can see this happening on large scales. The universe is expanding because of the big bang, but this expansion is accelerating due to dark energy!