Medium-sized black holes either don't exist or are very rare, say astronomers of the new study. The smallest known black holes formed from exploding supernovas, and are about 10 times larger than the Sun.Black holes in contrast, are billions of times larger than the Sun, and reside deep in the core of most galaxies. Middle of the road black holes are generally thousands of times larger than the Sun.The results are based on a comprehensive analysis of globular cluster RZ2109, using the European S..