Scientists have long imagined “supermassive” black holes at the center of many galaxies, but they never realized just how big they could be. Until now. A team of U.S. scientists has spotted the two biggest black holes ever discovered in the center of two galaxies that are near ours. Each of them is nearly 10 billion times the mass of the sun, and thousands of times bigger than the black hole that sits at the center of the Milky Way. Scientists believe that in the early days of the universe, quasars—galactic light sources—were powered by these massive black holes.