KISSIMMEE, FLORIDA--Most, if not all, galaxies have supermassive black holes at their centers surrounded by dense clouds of stars. Now, researchers have found one that seems to have lost almost its entire entourage. The team, which reported its find here today at the annual meeting of the American Astronomical Society, says it doesn't know what stripped the stars away. But it has put forward a tantalizing possibility: The object could be an extremely rare medium-sized black hole, which theorists have predicted but observers have never seen. The unusual black hole sits about 1 billion light-years from Earth in SDSS J1126+2944,...