At the heart of the Milky Way, just 27,000 light-years from Earth, there is a supermassive black hole with a mass of more than 4 million Suns. Nearly all galaxies contain a supermassive black hole, and many of them are much more massive. The black hole in the elliptical galaxy M87 has a mass of 6.5 billion Suns. The largest black holes are more than 40 billion solar masses. We know these monsters lurk in the cosmos, but how did they form? One idea is that supermassive black holes form over time