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 $χ$ 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 $ψ$ with suppressed pair-annihilation rates, that does not perturb the CMB, and whose late-time decays $ψ\rightarrow χ$ fill the universe with $χ$ 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.