As the universe cooled, the rest energy density of matter came to gravitationally dominate that of the photon radiation. The recombination epoch began after about 379,000 years, when the electrons and nuclei combined into atoms (mostly hydrogen), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background.
As the universe cooled, the rest energy density of matter came to gravitationally domi.nate that of the photon radiation. The recombination epoch began after about 379,000 years, w?hen the electrons and nuclei combined into atoms (mostly hydrogen), which were able to emit radiation. This relic radiation, which !continued through space largely unimpeded, is !known as the cosmic microwav?e background.