"A budding new paradigm is that black holes—in a dance of mutual self-regulation—may influence almost everything about galactic origins, growth, form and ultimate fates. They are not just the overstuffed kernels in the middle of galaxies. For reasons not fully understood, it appears that the sizes of central black holes and the masses of their galaxies, especially the central bulges, are almost perfectly in step.

The relation has become clear only since the late 1990s. Even the halo mass of dark matter—the mysterious invisible stuff that seems to make up more than 80 percent of all matter—around galaxies seems correlated with the size of supermassive black holes in galactic centers. That is a surprise. And when the black holes stop growing, galaxies themselves appear to stop evolving. "Now, we think we cannot understand galaxies without understanding black holes," says Abraham "Avi" Loeb, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass."