Intermediate and Primordial

Between stellar and supermassive lies a mysterious gap. Intermediate-mass black holes (100 to 100,000 solar masses) are the hardest to find. A few candidates turn up in dense star clusters, and gravitational wave detectors have caught mergers producing black holes in this range, like the 142 solar mass remnant from GW190521.

Primordial black holes are different: hypothetical objects that may have formed in the dense early universe, just fractions of a second after the Big Bang. They could be almost any size — from subatomic to thousands of solar masses — and some physicists think they might account for part of the universe's dark matter.