Cosmic background radiation; Primal glow
The background of radiation mostly in the frequency range 3 x 108 to 3 x 1011 Hz discovered in space in 1965. It is believed to be the cosmologically redshifted radiation released by the Big Bang itself.
Cosmic background radiation spectrum as determined with the COBE satellite, (top) uncorrected, (middle) corrected for the dipole term due to our peculiar velocity, (bottom) corrected for contributions from the dipole term and from our galaxy
Cosmic background radiation is electromagnetic radiation from the sky with no discernable source. It comes from the effects of interplanetary dust, and interstellar matter. The origin of this radiation depends on the region of the spectrum that we are observing. 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 detail observations of its properties are considered one of the major confirmations of the Big Bang.
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 individual source. See cosmic infrared background and X-ray background.
Source: wikipedia, NASA