  If the cosmological dark matter is primarily in the form of an elementary
particle which has cross section and mass for self-interaction having a ratio
similar to that of ordinary nuclear matter, then seed black holes (formed in
stellar collapse) will grow in a Hubble time, due to accretion of the dark
matter, to a mass range 10^6 - 10^9 solar masses. Furthermore, the dependence
of the final black hole mass on the galaxy velocity dispersion will be
approximately as observed and the growth rate will show a time dependence
consistent with observations. Other astrophysical consequences of collisional
dark matter and tests of the idea are noted.
