In the Big Bang theory, the era that started when the gravitational effect of matter began to dominate the effect of radiation pressure. Although radiation is massless it has energy, which means it has a gravitational effect; this effect increases with the intensity of the radiation. At high energies matter itself behaves like electromagnetic radiation because it is moving at a speed close to that of light. In the very early Universe, the expansion rate was dominated by the gravitational effect of radiation pressure but, as the Universe cooled, this effect became less important than the gravitational effect of matter. Matter is thought to have become predominant at a temperature of around 10⁴ K, roughly 30 000 years after the Big Bang. This marked the start of the matter era. See also Radiation era.