The origin of the cosmic, diffuse gamma ray background (EGB) is still an open question. Amongst observed astrophysical sources, the most promising candidate sources are star-forming galaxies, active galaxies and potentially others including pulsars. Only after evaluating contributions from these guaranteed sources can one accommodate contributions from exotic sources such as annihilating or decaying dark matter. In the light of detection of resolved star-forming galaxies by Fermi-LAT, their unresolved counterparts constitute an important component of the EGB as measured by LAT. Computation of the total contribution from both normal and starburst galaxies via various mechanisms to the EGB in the LAT energy range shows that the total gamma-ray emission from the star-forming universe could account for a significant fraction of EGB for the best case model.
Even less optimistic models remain an important contribution. The most dominant gamma ray production mechanism as established by LAT data analysis is from the hadronic channel involving neutral pion decay. A new calculation of leptonic gamma rays via inverse-Compton (IC) scattering of cosmic ray electrons shows it to be second most important though subdominant to pionic throughout the LAT range. The bremsstrahlung component is further lower. It may be possible to distinguish amongst the EGB candidates with the help of observables such as spectral features, statistics, anisotropy of sources, etc. These advances in our understanding of the origin of the EGB can put strong constraints on fundamental physics and astrophysics.