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This is where electrons, the bind with nuclei to form neutral atoms at this point. No longer nuclei, but they're now forming atoms, photons, parts in thermal equilibrium with no matter and the cosmos. The universe becomes transparent. The CMB, cosmic microwave background, photons originated this time, about 280,000 years after the big bang. And there's about 500 million hydrogen and helium atoms per per-rup meter cube at this point. The universe cooled at this point, the 2,000K electrons and protons that enter the recombine, hydrogen formed. The recombination, or recombining, is an incorrect name. They weren't recombining. They were actually combining.