Why Do We Think Most of the Universe is Missing?
About 380,000 years after the big bang, the universe cooled enough to allow protons and electrons to combine to form neutral hydrogen atoms. This is called recombination. Photons that had previously bounced around between free protons and electrons suddenly had nowhere to go and were released, flooding the universe with cosmic background radiation.
This is the third in a series of posts on the Oxford University Press TUMBLR site.
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