(CGB, DGRB, EGB, diffuse gamma ray background, extragalactic gamma ray background, diffuse gamma ray emission)
(gamma-ray radiation seen everywhere)
Cosmic gamma-ray background (CGB) radiation is weak gamma-ray
radiation detectable in many directions, from beyond the Milky Way.
The term diffuse gamma ray background (DGRB) is generally used
for the same thing.
By definition, it constitutes part of the cosmic background radiation (CBR).
Its source is of research interest, current theories attributing to
a variety of sources such as active galactic nuclei, blazars,
radio galaxies, star-forminggalaxies, and perhaps supernovae.
The source-types are presumed to vary in photon energy,
i.e., the lowest energies are produced by some types, and
the middle and higher energies by others.