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  1. en.wikipedia.org › wiki › UniverseUniverse - Wikipedia

    7 hours ago · According to the Big Bang theory, the energy and matter initially present have become less dense as the universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10 −32 seconds, and the separation of the four known fundamental forces , the universe gradually cooled and continued to expand, allowing the first subatomic particles and simple atoms to form.