  We propose to test the dark matter (DM) interpretation of the positron excess
observed by the PAMELA cosmic-ray (CR) detector through the identification of a
Galactic diffuse gamma-ray component associated to DM-induced prompt and
radiative emission. The goal is to present an analysis based on minimal sets of
assumptions and extrapolations with respect to locally testable or measurable
quantities. We discuss the differences between the spatial and spectral
features for the DM-induced components (with an extended, possibly spherical,
source function) and those for the standard CR contribution (with sources
confined within the stellar disc), and propose to focus on intermediate and
large latitudes. We address the dependence of the signal to background ratio on
the model adopted to describe the propagation of charged CRs in the Galaxy, and
find that, in general, the DM-induced signal can be detected by the Fermi
Gamma-ray Space Telescope at energies above 100 GeV. An observational result in
agreement with the prediction from standard CR components only, would imply
very strong constraints on the DM interpretation of the PAMELA excess. On the
other hand, if an excess in the diffuse emission above 100 GeV is identified,
the angular profile for such emission would allow for a clean disentanglement
between the DM interpretation and astrophysical explanations proposed for the
PAMELA excess. We also compare to the radiative diffuse emission at lower
frequencies, sketching in particular the detection prospects at infrared
frequencies with the Planck satellite.
