  We calculate the pair-annihilation cross section of real scalar singlet dark
matter into two mono-energetic photons. We derive constraints on the theory
parameter space from the Fermi limits on gamma-ray lines, and we compare with
current limits from direct dark matter detection. We show that the new limits,
albeit typically relevant only when the dark matter mass is close to half the
Standard Model Higgs mass, rule out regions of the theory parameter space that
are otherwise not constrained by other observations or experiments. In
particular, the new excluded regions partly overlap with the parameter space
where real scalar singlet dark matter might explain the anomalous signals
observed by CDMS. We also calculate the lifetime of unstable vacuum
configurations in the scalar potential, and show that the gamma-ray limits are
quite relevant in regions where the electro-weak vacuum is meta-stable with a
lifetime longer than the age of the universe.
