The scientists estimate that roughly 70 percent of the objects in that image are supermassive black holes, and in the whole image there are about 5,000 sources. Imagine: Thousands of black holes in just that one tiny part of the sky! Extrapolating to the whole sky, astronomers estimate there must be more than 1 billion supermassive black holes out in the deep Universe that Chandra could see. A billion.
That’s a lot of black holes. And it’s actually only a tiny percentage of what’s out there; there are hundreds of billions or even trillions of galaxies in the Universe. Each may have its own central black hole, but we just don’t see them (because they’re quiet, or feeding but still too faint to see at large distances).
And that’s just the supermassive black holes. Ones with lower mass, formed when stars explode, probably number in the many millions per galaxy. Extrapolating that means there are quadrillions of black holes in the visible Universe.