Figure 14-13Evolution of Mass/Energy Density (ρ) For approximately 30,000 years after the Big Bang, the gravitational effects created by photons (ρ_rad, shown in red) exceeded the gravitational attraction of all the matter in the universe (ρ_m, shown in blue). This early period is said to have been radiation-dominated. Later, however, continued expansion of the universe caused ρ_rad to become less than ρ_m, at which time the universe became matter-dominated.