Cosmic background radiation
Cosmic background radiation is electromagnetic radiation from the sky with no discernible source. The origin of this radiation depends on the region of the spectrum that is observed. One component is the cosmic microwave background radiation. 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 and detailed observations of its properties are considered one of the major confirmations of the Big Bang. The discovery (by chance in 1965) of the cosmic background radiation proves that the early universe was dominated by a radiation field, a field of extremely high temperature and pressure.
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.