Astronomers have been puzzling over a mystery for a few years now, and they think they might have an answer. Everywhere you look, there's a diffuse glow of background X-ray radiation - but where's it coming from? Astronomers pointed the Chandra X-Ray Observatory at a patch of sky for a total of 23 days over a 2-year period, and resolved 600 separate point sources. This isn't a background glow at all, but the X-ray radiation from hundreds of millions of supermassive black holes, like the one at the heart of the Milky Way.