Astronomers believed that at the center of essentially every galaxy is a supermassive black hole (supermassive = more material than a million Suns. Some are believed to have a mass more than a billion times that of the Sun). Where these black holes come from, no one really knows, but they gain mass by accreting (think swallowing) the surrounding gas. This gas forms a disk as it falls into the black hole, in which gas particles can stick together to form molecules - which astronomers call "dust." The older the disk, the more dust that should be there - or so goes the current thinking. By this logic, using Spitzer astronomers just found a couple supermassive black holes with very young disks. Go here to read more. Not necessarily the most convincing argument, but interesting nevertheless.