After exhausting their nuclear fuel, stars with masses approximately 3 times the mass of our sun or larger, which make up a relatively small fraction of stars, are believed to collapse to densities high enough so that escape velocities near the stars become higher than c. These collapsed stars are called black holes. Dozens of possible black holes have been observed and a typical galaxy is thought to contain on the order of 10 to 100 million of them. The universe itself is thought to contain at least 10¹⁸ (a million million million) black holes.

Black holes can also be created through processes other than stellar collapse. Smaller mass black holes may have formed under conditions found in the early universe. Supermassive black holes, with masses millions or billions of times that of our sun, might form when a large number of stars lying within a small region of space merge, from the growth of a smaller seed black hole, or when a large number of black holes merge. Supermassive black holes seem to reside at the center of nearly all galaxies and are theorized to play a very important role in galaxy formation. Recent work indicates that there may also be intermediate mass black holes that naturally form in star-forming regions.