Because light has a long way to travel, when you look at the stars at night, you are really looking at the stars from thousands or even billions of years ago. Astronomers are able to use this to tell that the universe was much different billions of years ago and has been constantly changing.
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Before the planets and stars formed, the universe was hotter, denser, and filled with a glow made from white fog and hydrogen plasma. When the universe expanded and cooled, protons and electrons combined to form neutral atoms that could no longer absorb thermal radiation.
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