  A new estimation of the isotropic diffuse gamma-ray background (IGRB)
observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space
Telescope (Fermi) has been presented for 50 months of data, in the energy range
100 MeV-820 GeV and for different modelings of the Galactic foreground. We
attempt here the interpretation of the Fermi-LAT IGRB data in terms of the
gamma-ray unresolved emission from different extragalactic populations. We find
very good fits to the experimental IGRB, obtained with theoretical predictions
for the emission from active galactic nuclei and star-forming galaxies. In
addition, we probe a possible emission coming from the annihilation of weakly
interacting dark matter (DM) particles in the halo of our Galaxy. We set
stringent limits on its annihilation cross section into gamma rays, which are
about the thermal relic value for a wide range of DM masses. We also identify
regions in the DM mass and annihilation cross section parameter space which can
significantly improve the fit to the IGRB data. Our analysis is conducted
within the different IGRB data sets obtained from different models for the
Galactic emission, which is shown to add a significant ambiguity on the IGRB
interpretation.
