Since no one has directly observed one, how do we know that black holes truly exist?
We see very strong evidence right at the center of our own galaxy. Astronomers have seen massive stars fall toward some central object and whip around it, like a comet around the sun, and fly back out. They have weighed that central object by measuring how strongly it whips stars around it. It turns out to have the same gravitational pull as approximately 3 million suns, and it is very dark—astronomers see only weak radio waves there. It almost certainly is a black hole. And when quasars [extremely bright, compact objects at the centers of some galaxies] were discovered in the early 1960s, it was obvious that the source of power had to be gravitational because even nuclear power, which powers the stars, is too inefficient. The idea that quasars are powered by the accretion of matter onto black holes was proposed within months after the discovery of quasars. This was a huge change of people’s views of the universe, and it came very quickly. There followed a period of rapid research, and
