Cosmic Dark Age 380 thousand to 150 million years

For many thousands of years nothing much changed. The Universe was still filled with fog as photons bounced off electrons. Electrons were attracted towards protons and helium nuclei because of their electrical charge, but they were all moving so quickly that when they met they just bounced off each other.

The Universe continued to expand and cool. Particles gradually moved slower and slower as they lost energy. Then began a process during which the fog cleared and the Universe became clear. This was the beginning of the time known as the Dark Age of the Universe. So what caused this dramatic change?

Recombination and Decoupling

After thousands of years of expansion and cooling, electrons were going so slowly that they were able to stick to protons to make entirely new types of object called atoms. This was the start of a dramatic change, a change which rapidly brought the Photon Epoch to an end about 380,000 years after the creation.

This is called recombination. It is a strange name. It suggests the electrons and nuclei combined again, but in fact they have never before been together.

Thinking about the separation of electrons from radiation, the event can also be called decoupling.