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Cosmic infrared background (CIB or CIBR) radiation is the infrared analog to cosmic microwave background (CMB), in the sense of there being infrared signal detected in areas between the known sources from beyond the Milky Way. By definition, it constitutes part of the cosmic background radiation (CBR). Theory suggested redshifted galaxies and galaxy clusters should create such radiation. IRAS first observed it and COBE has been used to study it. It is of interest in estimating the number and strength of light sources in early cosmic history. In addition to highly redshifted light sources, cosmic dust heated by light sources also contributes to it. The CMB, under the Sunyaev-Zel'dovich effect (SZ effect), also can reach infrared, creating a challenge distinguishing the SZ effect from CIB sources.