Michael Schirber in Space.com:
Though invisible, big black holes are not hard to find. Astronomers have noted evidence in the center of many galaxies for supermassive black holes weighing millions to billions of times our Sun.
Where these huge holes came from is an open question. One theory is that they are the result of a progressive build-up of smaller black holes, starting from the stellar mass black holes that formed from the explosions of the first stars.
If this hierarchical formation is true, then some of the middle stages between the 10-solar-mass acorns and the billion-solar-mass oaks should still be around. Yet confirmation of these intermediate mass black holes has been difficult to come by.