Sub-GeV Dark Matter Shining at Future MeV Gamma-Ray Telescopes
  We propose a novel framework where light (sub-GeV) dark matter (DM) is
detectable with future MeV gamma-ray telescopes without conflicting with Cosmic
Microwave Background (CMB) data. The stable DM particle $\chi$ has a very low
thermal relic abundance due to its large pair-annihilation cross section. The
DM number density is stored in a heavier, meta-stable partner $\psi$ with
suppressed pair-annihilation rates, that does not perturb the CMB, and whose
late-time decays $\psi \rightarrow \chi$ fill the universe with $\chi$ DM
particles. We provide explicit, model-independent realizations for this
framework, and discuss constraints on late-time decays, and thus on parameters
of this setup, from CMB, Big Bang Nucleosynthesis, and Large Scale Structure.
