Cosmic background radiation is electromagnetic radiation from the birth of the known universe. The origin of this radiation depends on the region of the electromagnetic spectrum that is observed. One component is the cosmic microwave background. This component is redshifted photons that have freely streamed from an epoch when the Universe became transparent for the first time to radiation. Its discovery (by chance in 1965) of cosmic background radiation suggests that the early universe was dominated by a radiation field of extreme high temperature and pressure.
The Sunyaev–Zel'dovich effect shows the phenomena of radiant cosmic background radiation interacting with "electron" clouds distorting the spectrum of the radiation. There is also background radiation in the infrared, x-rays, etc., with different causes, and they can sometimes be resolved into an individual source. See cosmic infrared background and X-ray background. See also cosmic neutrino background and extragalactic background light.