Since the 1960s, astronomers have uncovered evidence that most galaxies contain so-called supermassive black holes at their cores. With masses between a million and a billion times that of the Sun, these leviathans first revealed their presence in so-called quasars – distant galaxies with cores so luminous the only plausible source of power is the intense gravity of black holes devouring matter.
Since then, studies of stellar orbits have shown that even relatively tranquil galaxies like our own Milky Way harbour hefty black holes. Their origin remains a mystery, however. They may have been created by the gravitational collapse of giant gas clouds from which galaxies were formed, or from the merger of many smaller black holes over time.
Another possibility is that one simply grew over billions of years by steadily devouring orbiting stars.