Following the initial Big Bang, the universe was at incredibly high temperatures but then underwent a period of rapid expansion and cooling and largely became a cold gas of hydrogen. The universe could be considered to be dark at this time since there was no source of light. But at some point, matter started to clump together under the force of gravity and form embryonic stars and the hydrogen atoms started to fuse into helium, releasing energy, some of it in the form of visible light. This moment is referred to as the ‘cosmic dawn’ of the universe.
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