Cosmic background radiation
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. The most famous 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 Zeldovic theory shows the phenomena of radiant cosmic background radiation interacting with "electron" clouds distorting the spectrum of the radiation.
Other: List of astronomical topics