The Diffuse Gamma-Ray Flux from Clusters of Galaxies
  The origin of the diffuse gamma-ray background (DGRB) detected by EGRET and
Fermi-LAT, the one that remains after subtracting all individual sources from
the observed gamma-ray sky, is unknown. The DGRB possibly encompasses
contributions from different source populations such as star-forming galaxies,
starburst galaxies, active galactic nuclei, gamma-ray bursts, or galaxy
clusters. Here, we combine cosmological magnetohydrodynamical simulations of
clusters of galaxies with the propagation of CRs using Monte Carlo simulations,
in the redshift range $z\leq 5.0$, and find that the integrated gamma-ray flux
from clusters can contribute up to $100\%$ of the DGRB flux observed by
Fermi-LAT above $100$~GeV, for CR spectral indices $\alpha = 1.5 - 2.5$ and
energy cutoffs $E_{\text{max}} = 10^{16} - 10^{17}$~eV. The flux is dominated
by clusters with masses $10^{13}< M/M_{\odot} < 10^{15}$ and redshift $ z \leq
0.3$. Our results also predict the potential observation of high-energy gamma
rays from clusters by experiments like HAWC, LHAASO, and even the upcoming CTA.
