Half an hour after the Big Bang, the young universe was filled with hydrogen nuclei and electrons, but it was still much too hot for the nuclei to capture the electrons and form stable, neutral hydrogen atoms.
Light just bounced around in this plasma. Around the 378,000-year mark, though, atoms formed -- and the universe became transparent, with light finally able to travel long distances.
The light from this time is stretched out and cooler now, but we can still see it. What do we call it?