The cosmic microwave background radiation (CMB) is the residual electromagnetic radiation left over from the early universe, which has cooled and stretched over billions of years due to the expansion of space. It fills the universe uniformly and is observed in the microwave portion of the electromagnetic spectrum with a nearly uniform temperature of about 2.7 Kelvin. The discovery of the CMB in 1965 provided strong evidence for the Big Bang theory and has since been studied in detail to extract information about the composition, age, and geometry of the universe in its infancy.
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