The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology.
The CMB is a cosmic background radiation that is fundamental to observational cosmology because it is the oldest light in the universe, dating to the epoch of recombination.
With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark.
However, a sufficiently sensitive radio telescope shows a faint background glow, almost isotropic, that is not associated with any star, galaxy, or other object.
This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned the discoverers the 1978 Nobel Prize.
NOTE: Above information has been taken from wikipedia and/or official websites of topics.
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