Discovery of Cosmic Background Radiation
In 1965 Arno A. Penzias and Robert W. Wilson of Bell Laboratories were testing a sensitive horn antenna which was designed for detecting low levels of microwave radiation. They discovered a low level of microwave background "noise", like the low level of electrical noise which might produce "snow" on a television screen. After unsuccessful attempts to eliminate it, they pointed their antenna to another part of the sky to check whether the "noise" was coming from space, and got the same kind of signal. Being persuaded that the noise was in their instrument, they took other, more sophisticated steps to eliminate the noise, such as cooling their detector to low temperatures.
Finding no explanations for the origin of the noise, they finally concluded that it was indeed coming from space, but that it was the same from all directions. It was a distribution of microwave radiation which matched a blackbody curve for a radiator at about 2.7 Kelvins.
After all their efforts to eliminate the "noise" signal, they found that a group at Princeton had predicted that there would be a residual microwave background radiation left over from the Big Bang and were planning an experiment to try to detect it. Penzias and Wilson were awarded the Nobel Prize in 1978 for their discovery.
Blackbody radiation concepts