We know there are black holes at the center of galaxies because we can see them with radio telescopes and infrared telescopes. These telescopes can detect light waves that spiral in circles and in the middle of that circle is a massive hole that is black. We can also see stars orbiting this thing at extremely fast speeds. When we do calculations, we can determine that this black object has extreme mass due to the speed that the surrounding stars orbit it. The faster the star orbit, the more massive the black hole.
