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 slightly heavier, metastable partner $\psi$ with suppressed pair-annihilation rates, that does not perturb the CMB, and whose late-time decays $\psi \to \chi$ 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.

• Received 12 June 2018
• Revised 16 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.071101

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Published by the American Physical Society