  By using recent data, we directly determine the dark matter (DM) induced
$e^\pm$ spectrum at the source from experimental measurements at the earth,
without reference to specific particle physics models. The DM induced gamma
rays emitted via inverse Compton scattering are then obtained in a model
independent way. However the results depend on the choice of the astrophysical
$e^\pm$ background, which is not reliably known. Nevertheless, we calculate, as
an illustration, the fluxes of gamma rays from the Fornax cluster in the
decaying DM scenario with various astrophysical $e^\pm$ backgrounds. Without
any assumptions on details of the DM model, the predictions turn out to be
either in disagreement with or only marginally below the upper limits measured
recently by the Fermi-LAT Collaboration. In addition, these DM induced ICS
gamma rays in the GeV range are shown to be almost independent of choices of
cosmic ray propagation model and of DM density profile, when a given
astrophysical $e^\pm$ background is assumed. This provides a strong constraint
on decaying DM scenario as the gamma rays may be produced in other processes
besides inverse Compton scattering, such as the bremsstrahlung and neutral pion
decays.
