Where are all the intermediate mass black holes? Microlensing fast radio bursts might reveal them

Astrophysicists think that black hole masses are hierarchical. The largest are supermassive black holes (SMBH) like the one at the center of the Milky Way and other galaxies. Stellar mass black holes are born of collapsing stars, and are smaller. The smallest of all are the theoretical primordial black holes, which only formed in the weird physics of the early universe.

Raccontata daphys.org