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"cosmic microwave background (CMB), electromagnetic radiation filling the universe that is a residual effect of the big bang 13.8 billion years ago. Because the expanding universe has cooled since this primordial explosion, the background radiation is in the microwave region of the electromagnetic spectrum."
https://www.britannica.com/science/cosmic-microwave-background
Cosmic Microwave Background Explained
Secrets of the Cosmic Microwave Background