We know it’s there, we just don’t know what it’s made of
Dark matter is out there. It’s invisible. It’s called dark because it doesn’t interact with light. So the way we see things is, basically, you shine light on them, or hit them with a particle, and expect to see something bounce back that you can measure. Dark matter doesn’t interact through the electromagnetic force, ah, so it’s dark in the sense that we’ve got no way of catching it or capturing it other than with gravity. But gravity is the weakest of the four forces so it’s much more difficult to see dark matter.
We know it’s there because it’s the stuff that holds galaxies together. If you count up, and add up all the stuff that we can see, normal matter, in galaxies: the stars, the planets, the interstellar dust and gas, there’s just not enough stuff for its combined gravitational pull to hold the galaxies together. Now, galaxies tend to spin, they rotate very slowly, with spiral arms. If there was no dark matter then things would just drift off. They wouldn’t be able to be held in orbit around the galactic centre. So dark matter is there and it must provide the gravitational glue that holds galaxies together.
We know it’s there, we just don’t know what it’s made of and we’re hopefully zooming in on what its constituents are. Clearly it’s going to be made up of some kind of new particle, with new properties that we’ve yet to learn about. So one hopes that at the Large Hadron Collider, now that we’ve found the Higgs, we’ll move on and maybe discover some other new particle – or particles – that make up dark matter.
Next month Professor Jim Al-Khalili returns to break down dark energy.