How do we know that black holes actually exist?

Since no light can escape from a black hole, it’s impossible to see them, but several clues give away their existence. A black hole’s powerful gravitational field pulls on nearby stars, affecting their motion and allowing astronomers to infer that they must be orbiting a black hole. As gases spiral closer to a black hole they heat up to millions of degrees, emitting X-ray radiation that can then be detected by space telescopes.