An optical image of a distant galaxy, with the X-ray sources spotted by the Chandra Observatory outlined in white. Observations show that black holes at the centers of galaxies are the likely source of a faint X-ray glow filling the universe. They come in two, very different types. The smaller type are collapsed stars, typically a few times more massive than the sun. The other are supermassive black holes that lie at the center of many, if not most, galaxies, weighing millions to billions of times more than the sun.
Black holes are the answer.
It didn't find any that close. This was the very first result of X-ray astronomy. An article describing the findings has been submitted to the journal Nature. The researchers peered at a small piece of the sky, about one-fifth the area of a full moon. Once Chandra spotted where the X-rays were coming from, the Hubble Space Telescope and the Keck telescope in Hawaii took a look to try to figure out what was producing them.