What is the relic radiation from the Big Bang called, which is spread throughout space but shifted out of the visible spectrum?

Answer

The Cosmic Microwave Background (CMB)

The Cosmic Microwave Background (CMB) is the ancient light from the universe's hot, dense beginning, which has been stretched by expansion into the microwave range, making it invisible to human eyes.

What is the relic radiation from the Big Bang called, which is spread throughout space but shifted out of the visible spectrum?
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