The era between about 150 million and 1 billion years after the Big Bang, when the Universe went from consisting of neutral atoms to once again being an ionized plasma as it had been immediately after the Big Bang. This epoch is seen as the next major step in the development of the Universe following the recombination epoch. The recombination epoch occurred about 300 000 years after the Big Bang, when the Universe had cooled sufficiently for the positive ions and negative electrons to combine into neutral atoms. However, once the first stars were formed their ultraviolet radiation began to ionize the atoms again, hence initiating the reionization epoch. Several hundred million years would have been needed for the intergalactic medium to become fully ionized, and it remains so today.