When any patch of the sky is observed where no individual sources can be discerned, and the effects of interplanetary dust, and interstellar matter are taken into account, there is still radiation.

This radiation is known as Cosmic Background Radiation. The origin of this radiation depends on the region of the spectrum that we are observing. Certainly the most famous component is the Cosmic microwave background radiation. In the standard model, this is taken to be a remainder of the epoch when the universe, still hot, became transparent for the first time to radiation. The Sunyaev Zeldovic theory shows the phenonomena that 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; most of these are ultimately attributable to unresolved individual sources.

See also

Main: Timeline of cosmic microwave background astronomy, Cosmic microwave background radiation, Galaxy formation and evolution, Magnitude, Universe

Cosmology: Big Bang, Non-standard cosmology

Physics: Dark matter, Dirac sea, Hot dark matter, Irradiation, Sunyaev Zeldovic Effect