About 14 billion years ago, the Universe began as an explosion (ie. the “Big Bang”) from a single, infinitely dense point. Formation of helium and deuterium nuclei occurred within about the first 10 minutes, but actual atoms of helium and hydrogen did not form until the Universe was about 300,000 years old, when things cooled to a few thousand °C. The Universe continued to expand and cool from there, with proto-galaxies beginning to form within the first billion years. Big Bang
Periodic Table of the Elements
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