Is there a large population of undiscovered dark galaxies?
Astronomers have long known that small galaxies are much more numerous than large ones. The faintest are believed to be the most numerous galaxies in the Universe. Yet we continue to find more. We expect that we have not discovered all of them. Recent work also shows that smaller galaxies are much more dominated by dark matter. Our Milky Way is a large galaxy, and its main body is about 50% dark. Galaxies with 1/100 of the luminosity of the Milky Way are about 90% dark. The smallest dwarfs that we know about are almost completely dark: only 1% of their matter is in the form of stars. Astronomers believe that they understand why: less massive galaxies have a weaker gravitational hold on their contents, so the first stars that die in supernova explosions eject more of the remaining gas in smaller galaxies. These explosions have little effect on the dark matter. So small galaxies have less gas with which to make stars and therefore low stellar densities despite their high dark matter dens
