Schematic diagram of the history of the Universe

This diagram depicts the major milestones in the evolution of the Universe since the Big Bang, about 13.8 billion years ago. It is not to scale.

The Universe was in a neutral state at 400 thousand years after the Big Bang and remained that way until light from the first generation of stars started to ionise the hydrogen. After several hundred million years, the gas in the Universe was completely ionised.