  We compute the annihilation cross-section of two neutralinos at rest into two
photons, which is of importance for the indirect detection of neutralino dark
matter in the galactic halo through a quasi-monochromatic gamma-ray line. We
include all diagrams to one-loop level in the minimal supersymmetric extension
of the standard model. We use the helicity formalism, the background-field
gauge, and an efficient loop-integral reduction method. We confirm the result
recently obtained by Bergstrom and Ullio in a different gauge, which disagrees
with other published calculations.
