The Extragalactic Gamma Ray Background

One way to understand the nonthermal history of the universe is by establishing the origins of the unresolved and truly diffuse extragalactic gamma rays. Dim blazars and radio/gamma galaxies certainly make an important contribution to the galactic gamma-ray background given the EGRET discoveries, and previous treatments are reviewed and compared with a new analysis. Studies of the gamma-ray intensity from cosmic rays in star-forming galaxies and from structure formation shocks, as well as from dim GRBs, are briefly reviewed. A new hard gamma-ray source class seems required from the predicted aggregate intensity compared with the measured intensity.