400 00 years after the Big Bang, matter cooled, and the Universe is filled almost of neutral hydrogen atoms. This first phase, called "dark age" lasts several hundreds of millions of years. It is gradually ended when the first stars and galaxies formed and, according to several models, the hydrogen atoms of the intergalactic medium are ionized by the strong ultraviolet emission of the first generation of stars, called Population III, extremely massive stars (several tens of solar masses). The epoch of reionization of the Universe begins.
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