7.5 Outstanding issues
Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes.
Quasars were most abundant at redshifts of 2-3 and have been declining in number for the last 10 billion years.
It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenated as a result of galactic collisions or mergers.
Supermassive black holes found in the nuclei of the Milky Way and other galaxies may be the remnants of extinct AGNs.