The period after the big bang when the universe was sufficiently cool for electrons and nuclei to form atoms for the first time. The recombination occurred about 300 000 years after the big bang. Up to this time, photons in the universe were scattered by free electrons and it is sometimes said that the universe was ‘opaque’. With recombination, photons were able to travel without impediment, and the universe became ‘transparent’. See also microwave background radiation.