  We investigate the phenomenology of dark matter decay assuming that it is
induced by non-minimal coupling to gravity, when the dark matter mass is in the
sub-GeV range, i.e. below the QCD confinement scale. We show that the decay of
the singlet scalar dark matter candidate produces sharp features in the photon
spectrum, in the form of lines, boxes, and also in the form of a novel spectral
feature, characterized by the decay into $e^+e^- \gamma$ through a contact
interaction, with decay branching fractions depending only on a single
parameter, namely the dark matter mass. We also derive upper limits on the
strength of the gravity portal from the non-observation of sharp features in
the isotropic diffuse gamma-ray spectra measured by COMPTEL, EGRET and
Fermi-LAT, and the X-ray spectrum measured by INTEGRAL. Finally, we briefly
comment on the impact of dark matter decay via non-minimal coupling to gravity
on the reionization history of the Universe.
