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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-forming galaxies, 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.