Matter, radiation, dark energy: equality and dominance

What you are seeing: an expanding comoving patch whose contents change as the scale factor $a$ grows. Early on a hot photon gas dominates ($\rho_r \propto a^{-4}$: photons redshift and dilute fastest); past equality matter takes over ($\rho_m \propto a^{-3}$) and gravity switches on, so the matter clumps into structure; finally $\rho_\Lambda$ (constant) dominates. The lower strip plots the three components on a log-log scale with $a_{eq}$ and a marker at the current $a$.

Two crossovers are marked: matter-radiation equality at $a_{eq} = \Omega_r / \Omega_m \approx 2.9 \times 10^{-4}$ ($z_{eq} \approx 3410$), and matter-Lambda equality at $a_{m\Lambda} = (\Omega_m / \Omega_\Lambda)^{1/3} \approx 0.77$. Below $a_{eq}$ the universe is radiation-dominated; between $a_{eq}$ and $a_{m\Lambda}$ it is matter-dominated; today (just past $a_{m\Lambda}$) it is Lambda-dominated and accelerating.