Galaxies have probably been formed from fluctuations in density and motion in the universe. Many cosmologists have suggested that the separation into individual units took place around the time of decoupling of matter and radiation, when the radius of the universe was about 1/1000th of the present radius. In this case the protogalaxies must have obtained their angular momentum at a later stage in their evolution, through interaction with other protogalaxies or with galaxy clusters. For when the radius of the universe was 1/1000th of its present value, and the density about 10−20g cm−3, a mass of, say, 1011M⊙ would have had a radius of 10 cm. A mass with such a radius cannot contain anything like the angular momentum contained in a galaxy like the Milky Way system.