The Positron Puzzle

The Positron Puzzle concerns the origin and morphology of the Galactic 511 keV gamma-ray line, which shows an unusual bulge-to-disk luminosity ratio near unity and requires an annihilation rate of ~5×10^43 e+ s−1 in the Milky Way. Positrons cool to eV energies, form positronium, and annihilate to produce a narrow 511 keV line and an ortho-positronium continuum. This review clarifies what is known and unknown about the signal’s spatial distribution and spectral properties, identifies where data and simulations have been used inadequately leading to spurious claims, and synthesizes current constraints on positron sources, propagation, and annihilation conditions. It evaluates candidate sources ranging from nucleosynthesis products and compact objects to dark matter and stellar flares, discusses transport from MeV to GeV scales, and outlines observational and analysis strategies to resolve the puzzle.