Cosmic microwave background constraints on dark matter models of the Galactic center 511 keV signal

The high positron production rate required to explain the flux of 511 keV gamma rays from the Galactic center has inspired many models in which dark matter creates positrons. These models include the annihilation of light dark matter and the scattering of dark matter with excited states (exciting dark matter). We show that existing cosmic microwave background data robustly constrain such models when the annihilation or scattering cross section is not velocity suppressed depending on the model of the Galactic dark matter halo. Upcoming data from the Planck mission can exclude the fiducial Via Lactea II halo model, which also provides a good fit to the 511 keV morphology. We additionally find combined constraints on exciting dark matter scattering and annihilation, and update constraints on the lifetimes of dark matter excited states. Finally, we apply constraints to models of dark matter decay in which produced positrons fall into the Galactic center and produce the 511 keV signal on their annihilation, demonstrating that most of the parameter space of interest is ruled out.