Theory and phenomenology of dark matter models for the 511 keV galactic photons

The origin of the 511 keV gamma-ray line from the Galactic Center has been one of the biggest misteries and most enduring puzzles in astrophysics since its discovery. While conventional astrophysical explanations struggle to account for both the intensity and morphology of the signal, models involving light dark matter have emerged as promising candidates over the past decades. In this thesis we investigate dark matter models that are able to consistently explain the 511 keV line from the Galactic Center, going through their theoretical foundation and analyzing their phenomenological consequences. The core of this work focuses on two specific frameworks: one involving p-wave annihilation of dark matter particles and another dealing with coannihilation between nearly-degenerate dark matter states. We examine the compatibility of these models with experimental and observational constraints, such as those from direct detection experiments and collider searches, along with bounds from the Cosmic Microwave Background (CMB), Supernova 1987A (SN 1987A) and observations of the Bullet Cluster. The results obtained in this master thesis show that viable dark matter scenarios can explain the 511 keV line and reproduce the observed dark matter relic abundance, while remaining compatible with current experimental limits. Our analysis also identifies clear strategies to test these potential dark matter explanations of the 511 keV line in the near future, using upcoming experiments with enhanced sensitivity to sub-GeV dark matter.