Over the years, astronomers have readily detected stellar black holes, which are relatively small and equal in mass to a few Suns or less, and supermassive black holes, which are equal in mass to millions of Suns. However, black holes of intermediate mass have notably eluded detection, prompting scientists to theorize why. New research suggests that intermediate-mass black holes might not have been discovered because they may not exist in our modern-day Universe for a very simple reason: the growth rate of black holes.
Scientists believe that stellar-mass black holes form when huge stars die and collapse inward. These are the “standard” black holes you might envision when you think of stars dying, or when you imagine the millions of black holes that dot our Universe. Supermassive black holes are those that form the hearts of large galaxies like our own Milky Way. To date, the oldest supermassive black holes found include a discovery from 2015 — a throwback to a much younger version of our Universe, when it was only about 875 million years old. The overall picture presented by our findings on supermassive black holes so far indicates that those early days of the Universe were friendlier for the formation of supermassive black holes, since matter was more concentrated in the much smaller Universe.