Black holes pack an extraordinary amount of mass into a small spatial volume, resulting in gravitational fields so powerful that not even light can escape. These extreme objects occur as two distinct populations in the universe. The first, called stellar black holes, form from the collapse of massive stars and can possess anywhere from a few to dozens of times the Sun's mass. The second population, supermassive black holes, are found usually as single (although occasionally as double) objects in the cores of most galaxies. As their name suggests, supermassive black holes contain millions to even billions of times the Sun's mass. Astrophysicists expect that a third population of black holes exists, bridging the substantial mass gap between the two well-documented black hole populations. This third population, known as intermediate-mass black holes (IMBHs), would have on the order of hundreds to thousands to even tens of thousands of solar masses. If confirmed, IMBHs would help solve the mystery of the formation of supermassive black holes. Depending on their mass and cosmic environments, IMBH origins could be “bottom-up,” involving the merging of many stellar black holes, or the merging of numerous massive stars within dense star clusters into a colossal object that quickly collapses into an IMBH. Alternatively, IMBHs could have a top-down genesis, arising from the collapse of huge amounts of available mass in the early universe. That top-down scenario is the favored—albeit unsubstantiated—formation model for supermassive black holes as well. Whichever way IMBHs appear to form would accordingly hone formation and evolution theories for supermassive black holes by potentially showing the lower-end mass levels that can arise from the top-down scenario or placing supermassive black holes alone in that origin scenario. See also: Black hole; Mass; Universe