- The time when matter and radiation first separated. The Universe became `transparent'; the cosmic microwave background was emitted. The 'decoupling' between matter and radiation happened because the Universe was cool enough already to allow protons to capture one electron and form a neutral atom of hydrogen: the electrons, charged particles that interact strongly with light, were not free anymore, and light could propagate. This happened some 300 000 years after the Big Bang.
See also: Big Bang Theory.