Early on, the whole universe was dense enough to be completely opaque. But at a time roughly 380,000 years after the Big Bang, expansion spread everything out enough to make the universe transparent. The light released from this transition, known as the cosmic microwave background (CMB), still exists.
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Early on, the whole universe was dense enough to be completely opaque. But at a time roughly 380,000 years after the Big Bang, expansion spread everything out enough to make the universe transparent. The light released from this transition, known as the cosmic microwave background (CMB), still exists.