Dark matter annihilation: The origin of cosmic gamma-ray background at 1-20 MeV

The origin of the cosmic gamma-ray background at 1-20 MeV remains a mystery. We show that gamma-ray emission accompanying annihilation of 20 MeV dark-matter particles explains most of the observed signal. Our model satisfies all of the current observational constraints, and naturally provides the origin of "missing" gamma-ray background at 1-20 MeV and 511 keV line emission from the Galactic center. We conclude that gamma-ray observations support the existence of 20 MeV dark-matter particles. Improved measurements of the gamma-ray background in this energy band undoubtedly test our proposal.