`e consider the possible expansion of clusters or superclusters of galaxies, of mean rest-mass density p ,immersedin a universe containing a gas of particles having xero rest mass and having energy density pc2. The universe may also contain dust. It is shown that when p > p the clusters or superclusters expane with the universe, but if p p the expansion is reduced in the ratio p/p . It therefore seems unlikely thai, clusters of galaxies could have formed prior to the epoch corresponding to the redshift r 2 if the pres ent value of p exceeds 10-29 g , unless most of this radiation was generated outside the clusters ane' subsequent to their formation. Superclusters, if they exist, will follow the expansion of a universe ol' even lower energy density.