  Astrophysical searches for gamma rays are one of the main strategies to probe
the annihilation or decay of dark matter particles. We present a new class of
distinct sub-GeV spectral features that generically appear in kinematical
situations where the available center-of-mass energy in such processes is just
above threshold to produce excited meson states. Using a Fisher forecast with
realistic astrophysical backgrounds, we demonstrate that for upcoming
experiments like e-ASTROGAM and ComPair these signals can turn out to be the
smoking gun in the search for particle dark matter.
