The nature of the Diffuse Gamma-Ray Background
  We review the current understanding of the diffuse gamma-ray background
(DGRB). The DGRB is what remains of the total measured gamma-ray emission after
the subtraction of the resolved sources and of the diffuse Galactic
foregrounds. It is interpreted as the cumulative emission of sources that are
not bright enough to be detected individually. Yet, its exact composition
remains unveiled. Well-established astrophysical source populations (e.g.
blazars, misaligned AGNs, star-forming galaxies and millisecond pulsars) all
represent guaranteed contributors to the DGRB. More exotic scenarios, such as
dark matter annihilation or decay, may contribute as well. In this review, we
describe how these components have been modeled in the literature and how the
DGRB can be used to provide valuable information on each of them. We summarize
the observational information currently available on the DGRB, paying
particular attention to the most recent measurement of its intensity energy
spectrum by the Fermi LAT Collaboration. We also discuss the novel analyses of
the auto-correlation angular power spectrum of the DGRB and of its
cross-correlation with tracers of the large-scale structure of the Universe.
New data sets already (or soon) available are expected to provide further
insight on the nature of this emission. By summarizing where we stand on the
current knowledge of the DGRB, this review is intended both as a useful
reference for those interested in the topic and as a means to trigger new ideas
for further research.
