Astronomers have known for more than a decade that nearly every large galaxy contains at its core an immense black hole—an object having such intense gravity that even light cannot escape. The death of stars can produce small black holes—with masses ranging from about three to 100 times the mass of the sun—but such stellar-mass black holes are tiny compared with the behemoths at the centers of galaxies, measuring millions to billions of solar masses.
These supermassive black holes pose major puzzles: Why are they so common in galaxies? Which came first—the galaxy or the hole? And how did they form in the first place?