Question: what is dark matter

Monique Henson answered on 10 Nov 2015:

I don’t know – in fact no human being on the planet knows. It’s one of the great mysteries in physics at the moment.

Even though we don’t know what it is, we do know a few things about it. We know it doesn’t interact with light. That means it doesn’t absorb, emit or reflect light, so we can’t see it. This makes it quite different to all of the other matter that we know of – the stuff that’s around us every day.

We do know that affects things through gravity. In fact, that’s how we know it exists. If it wasn’t for dark matter, our galaxy (the Milky Way) wouldn’t stay together. It wouldn’t have enough gravity to hold it together, so it would tear apart.

It turns out that most of the matter in the Universe is dark matter. It’s not just astronomers looking for dark matter either. There are big particle physics experiments deep underground all over the world looking for dark matter too (there used to be one in Yorkshire).