Clusters of galaxies are expected to be significant gamma-ray emitters
with contributions from both, annihilating dark matter (DM) and
radiative processes induced by cosmic-rays (CRs). We present
high-resolution hydrodynamical simulations of galaxy clusters at
different dynamical stages using self-consistent CR physics in
combination with radiative hydrodynamics. An improved spectral
description of the CRs allows us to reliably predict both the spatial
and spectral emission profiles from various CR processes. Besides
establishing an entirely new class of sources, gamma-ray observations
of clusters should open up a unique window for studying the physics of
high energy, non-thermal processes in cosmic large-scale environments.
Using a DM model that fits the recent electron and positron data from
Fermi, PAMELA, and HESS with remarkable precision, we predict an
observable level of gamma-rays from nearby galaxy clusters for the
Fermi satellite. In order not to overproduce the EGRET upper limit on
the gamma-ray emission from Virgo, we constrain the minimum mass of
substructures in dark matter halos to be three orders of magnitudes
larger than the expectation from cold dark matter scenarios in this
particular DM model. This demonstrates that high-energy gamma-ray
observations can act as a microscope for the smallest cosmological
structures.