The original idea of a black hole goes all the way back to 1783, when Cambridge scientist John Michell recognized that a massive enough object in a small enough volume of space would render everything — even light — unable to escape from it. More than a century later, Karl Schwarzschild discovered an exact solution to Einstein's General Relativity that predicted the same result: a black hole.
Both Michell and Schwarzschild predicted an explicit relationship between the event horizon, or the radius of the region from which light cannot escape, and the mass of the black hole as well as the speed of light. For 103 years after Schwarzschild, this prediction went untested. At long last, on April 10, 2019, scientists revealed the first-ever picture of a black hole's event horizon. Einstein's theory won again, as did all of science.