Why does the Big Bang theory predict that the cosmic microwave background should have an essentially perfect thermal radiation spectrum?

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The spectrum of 75 percent hydrogen and 25 percent helium is a perfect thermal radiation spectrum.

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This radiation came from the temperature of the Universe.

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The first stars that formed in the Universe must all have had the same temperature.

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The spectrum of pure hydrogen is a perfect thermal radiation spectrum.

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The dark matter in the Universe is a perfect thermal radiation spectrum."

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The light from all the stars and gas in the sky averaged over the entire Universe is a perfect thermal radiation spectrum.

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This radiation came from the heat of the Universe itself.

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