New 511 keV Line Data Provide Strongest sub-GeV Dark Matter Constraints

We explore the 511 keV emission associated with sub-GeV dark matter (DM) particles that can produce electron–positron pairs and form positronium after thermalizing. We use ∼16 yr of data from the Spectrometer on INTEGRAL to constrain DM properties, including the full positron propagation and losses, and the suppression of free electron density away from the Galactic plane. We show that the predicted longitude and latitude profiles vary significantly for different DM masses, unlike previous assumptions, and obtain the strongest limits on sub-GeV DM (from the MeV to a few GeV) so far, excluding cross sections down to 〈 σ v 〉 ≲ 10 ^−32 cm ^3 s ^−1 for m _χ ∼ 1 MeV and 〈 σ v 〉 ≲ 10 ^−26 cm ^3 s ^−1 for m _χ ∼ 5 GeV and lifetimes up to τ ≳ 10 ^29 s for m _χ ∼ 1 MeV and τ ≳ 10 ^27 s for m _χ ∼ 5 GeV for the typical Navarro–Frenk–White DM profile. Our derived limits are robust within a factor of a few due to systematic uncertainties.