The Universe is full of radiations of all sorts—light, heat, infrared, ultra-violet, x-rays, gamma rays, cosmic rays, etc.—all emitted by specific objects. But in 1965 it was discovered that pervading all these and coming equally from all directions in space was a very gentle background radiation of a few centimeters wavelength. This represents a temperature of 3 degrees Kelvin and, since it seems to exist everywhere, suggests that this is radiation from, and the temperature of, the very universe itself—a sort of surviving echo of the original Big Bang when the Universe formed and began expanding. The existence of the CMB (Cosmic Microwave Background) radiation was first predicted by George Gamow in 1948.

DIRBE Emissions Graph, K & L Bands, courtesy NASA