General Astronomy/Supermassive Black Holes
The general consensus today is that when galaxies form, the phenomenal concentration of matter at their cores leads inevitably to the formation of "supermassive" black holes (SMBH). Unlike a "normal" black hole that forms from stars only a little more massive than our Sun (stellar mass black holes), "supermassive" black holes range from hundreds of thousands to perhaps a billion or more times the mass of our Sun.
Contrary to image in movies and the media, black holes do not "suck" everything in and it is perfectly possible for stars and gas clouds to orbit around a black hole without being pulled in or destroyed. In fact, observations of the very rapid motions of stars near the center of the Milky Way have given the first proof that such a monster lurks at the heart of our own Galaxy. The stars in this region are orbiting a common center, and their very fast motions indicate an extremely strong gravitational attraction at that point. Using Newton's version of Kepler's laws, astronomers have determined that this mass is the equivalent of about 4 million times the mass of our Sun. Since there is no light or other radiation coming from this point, we reach the inevitable conclusion that it is a supermassive black hole.
Other evidence strongly suggests that similar SMBHs exist in the cores of most if not all other galaxies. Ours is quiet or quiescent today, but not so in some other galaxies.