Asked by John Barnes
Because the early universe was hot and dense, there was a lot of energy. Meaning that pairs of particles crashed into each other to the point of destruction. As particles destroy each other, we get packets of pure energy, or particles of light known as photons.
As the universe grew older, it expanded, the temperature fell and the last particles and antiparticles destroyed each other, leaving us with about a billion photons for each and every particle of matter. We are now left with the universe that we see in our telescopes today.
The energetic photons in the cosmic microwave background were made in the first minute or so of the universe’s eventful history, cooling with the expansion of the universe, it is now at a rather chilly temperature of approximately -270 degrees Celsius (-454 degrees Fahrenheit).
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