Constraining dark matter annihilation with the isotropic $\gamma$-ray
  background: updated limits and future potential
  The nature of the Isotropic $\gamma$-ray Background (IGRB) measured by the
Large Area Telescope (LAT) on the Fermi $\gamma$-ray space Telescope ({\it
Fermi}) remains partially unexplained. Non-negligible contributions may
originate from extragalactic populations of unresolved sources such as blazars,
star-forming galaxies or galactic milli-second pulsars. A recent prediction of
the diffuse $\gamma$-ray emission from Active Galactic Nuclei (AGN) with a
large viewing angle with respect to the line-of-sight (l.o.s.) has demonstrated
that this faint but numerous population is also expected to contribute
significantly to the total IGRB intensity. A more exotic contribution to the
IGRB invokes the pair annihilation of dark matter (DM) weakly interacting
massive particles (WIMPs) into $\gamma$-rays. In this work, we evaluate the
room left for galactic DM at high latitudes ($>10^\circ$) by including photons
from both prompt emission and inverse Compton scattering, emphasizing the
impact of the newly discovered contribution from misaligned AGN (MAGN) for such
an analysis. Summing up all significant galactic and extragalactic components
of the IGRB, we find that an improved understanding of the associated
astrophysical uncertainties is still mandatory to put stringent bounds on
thermally produced DM. On the other hand, we also demonstrate that the IGRB has
the potential to be one of the most competitive {\it future} ways to test the
DM WIMP hypothesis, once the present uncertainties are even slightly reduced.
In fact, if MAGN contribute even at 90% of the maximal level consistent with
our current understanding, thermally produced WIMPs would be severely
constrained as DM candidates for masses up to several TeV.
