The center of the galaxy annihilates half a trillion trillion trillion positrons every second, and after fifty years no one knows where they come from. The 511 keV line, the signature of electron-positron annihilation, was first detected from the Galactic Center region in the 1970s and mapped by INTEGRAL/SPI: the emission is dominated by a bright bulge component with a fainter disk, requiring the annihilation of about 2 x 10^43 positrons per second. The puzzle is the morphology: known positron factories, massive stars, supernovae, pulsars, X-ray binaries, live mostly in the disk, while the signal concentrates in the bulge, with a bulge-to-disk ratio no conventional source census reproduces.
The mystery made it a dark matter target: light dark matter annihilating or de-exciting into electron-positron pairs could paint exactly a bulge-dominated profile. But the parameter space for MeV-scale dark matter is squeezed by cosmological and laboratory constraints, the line width and positronium fraction demand the positrons annihilate at low energies after substantial propagation, and no independent signature of the required particle has appeared. Conventional proposals, old microquasar outbursts, Sgr A* activity in the past, propagation of disk positrons into the bulge along magnetic fields, each cover part of the morphology, none commands consensus, and the propagation distances of MeV positrons in the inner galaxy remain too uncertain to settle attribution.
The standing is astronomy's oldest unexplained line: source unknown after five decades, dark matter interpretations alive but cornered, and the COSI mission (launching 2027) designed specifically to map the 511 keV sky with the resolution the attribution requires.