What mass class lies between stellar black holes and supermassive black holes?

An intermediate-mass black hole lies between stellar black holes and supermassive black holes.

These objects are commonly placed at roughly 100 to 100,000 solar masses, although boundaries vary between studies. Stellar black holes usually form from the collapse of massive stars, while supermassive black holes occupy galactic centers and reach hundreds of thousands to billions of solar masses.

IMBHs are important because they may represent a missing growth stage. Proposed formation routes include repeated mergers of stellar black holes, runaway stellar collisions in dense clusters, and primordial black holes formed in the early universe.

Evidence remains difficult to secure because an IMBH is faint unless it accretes matter. The 2019 gravitational-wave event GW190521 produced a remnant of about 142 solar masses, placing it in the intermediate-mass range and providing especially strong evidence for this population.