  The isotropic diffuse $\gamma$-ray background (IGRB) has been detected by
various experiments and recently the Fermi-LAT Collaboration has precisely
measured its spectrum in a wide energy range. The origin of the IGRB is still
unclear and we show in this paper the significative improvements that have been
done, thanks to the new Fermi-LAT catalogs, to solve this mystery. We
demonstrate that the $\gamma$-ray intensity and spectrum of the IGRB is fully
consistent with the unresolved emission from extragalactic point sources,
namely Active Galactic Nuclei and Star Forming Galaxies. We show also that the
IGRB can be employed to derive sever constraints for the $\gamma$-ray emission
from diffuse processes such as annihilation of Dark Matter (DM) particles. Our
method is able to provide low bounds for the thermal annihilation cross section
for a wide range of DM masses.
