Question: What is dark matter?

Nawapat Kaweeyanun answered on 15 Nov 2019:

When we take measurements of the universe, some of them came back a little … wrong. Take a disc-like galaxy for example. A star far away from the galaxy centre should move much slower than a star close to the centre, but it turns out they move at the same speed! But for this to be possible, the gravity we need is far greater than what provided by the matter we see. So there must be more matter in the universe! Since we cannot see them at all, we call these “dark matter”.