  We describe a characteristic signature of dark matter (DM) annihilation or
decay into gamma-rays. We show that if the total angular momentum of the
initial DM particle(s) vanishes, and helicity suppression operates to prevent
annihilation/decay into light fermion pairs, then the amplitude for the
dominant 3-body final state f^+f^-\gamma has a unique form dictated by gauge
invariance. This amplitude and the corresponding energy spectra hold for
annihilation of DM Majorana fermions or self-conjugate scalars, and for decay
of DM scalars, thus encompassing a variety of possibilities. Within this
scenario, we analyze Fermi LAT, PAMELA and HESS data, and predict a hint in
future Fermi gamma-ray data that portends a striking signal at atmospheric
Cherenkov telescopes (ACTs).
