The Anisotropy of the Extragalactic Radio Background from Dark Matter
  Annihilation
  Observations of the extragalactic radio background have uncovered a
significant isotropic emission across multiple frequencies spanning from 22 MHz
to 10 GHz. The intensity of this non-thermal emission component significantly
exceeds the expected contribution from known astrophysical sources.
Interestingly, models have indicated that the annihilation of dark matter
particles may reproduce both the flux and spectrum of the excess. However, the
lack of a measurable anisotropy in the residual emission remains challenging
for both dark matter and standard astrophysical interpretations of the ARCADE-2
data. We calculate the expected synchrotron anisotropy from dark matter
annihilation and show that these models can produce very small anisotropies,
though this requires galaxy clusters to have large substructure contributions
and strong magnetic fields. We show that this constraint can be significantly
relaxed, however, in scenarios where electrons produced via dark matter
annihilation can be efficiently reaccelerated by Alfv\'en waves in the
intra-Cluster medium. Our analysis indicates that any source capable of
explaining the intensity and isotropy of the extragalactic radio excess must
have a spatial extension far larger than typical for baryons in galaxies,
suggesting a novel physics interpretation.
