Reionization

After the initial explosion of energy and matter known as the Big Bang, the universe began to cool. About 300,000 years after the Big Bang, the universe had cooled so much that the hydrogen that fills it turned opaque to ultraviolet light. The universe continued to cool as it expanded until, somewhere between 300 million years to 1 billion years after the Big Bang, it was suddenly reheated by a release of energy that caused hydrogen to again become transparent to ultraviolet light. This reheating process is known as reionization.