Complex sentence: With photons decoupled from matter, the energy density of the Universe entered the matter-dominated era, and light, which could now travel freely can still be seen in the Universe as the cosmic microwave background (CMB).
Split into simpler sentences: With photons decoupled from matter, the energy density of the Universe entered the matter-dominated era. Light from this era could now travel freely, and it can still be seen in the Universe as the cosmic microwave background (CMB).