Question: how do you know where to look for black holes?

Claire Greenwell answered on 11 Mar 2019:

You can see them by looking at the things around them. Black holes have a lot of mass, so other objects in space can orbit around, and we can track these orbits to work out where the black hole is, and how big it is. They can also be big enough that light gets bent round them – this is called gravitational lensing. So if we look at something bright that should be behind a black hole, we’ll see it in a different place to where we expect because the light coming from it has been bent by the black hole.
Black holes often have a lot of gas and dust falling into them, and as it falls it can get very hot and bright and we can often see the radiation from this.