But perhaps the most significant milestone in the Universe’s evolution, at least in terms of detailed testing of cosmological predictions, occurred somewhat later on—about 380,000 years after the Big Bang. The Universe was originally filled with both charged and uncharged particles. But at this later time, the Universe had cooled sufficiently that positively charged nuclei combined with negatively charged electrons to form neutral atoms. From that time forward, the Universe consisted of neutral matter, which is matter that carries no electric charge.
