Figure 25-10: The Evolution of Density For approximately 24,000 years after the Big Bang, the mass density of radiation (ρrad, shown in red) exceeded the matter density (ρm, shown in blue), and the universe was radiation-dominated. Later, however, continued expansion of the universe caused ρrad to become less than ρm, at which point the universe became matter-dominated.
(Graph courtesy of Clem Pryke, University of Minnesota)