  For the first time, a full one-loop calculation of the process $\chi\chi\to
2\gamma$ is performed, where $\chi$ is the lightest neutralino in the minimal
supersymmetric extension of the Standard Model. This process is of interest for
dark matter detection, since it would give a sharp $\gamma$ ray line with
$E_\gamma=m_\chi$. We improve upon and correct published formulas, and give
cross sections for supersymmetric models with $\chi$ masses between 30 GeV and
several TeV. We find a new contribution, previously neglected, which enhances
the $2\gamma$ rate for TeV higgsinos by up to an order of magnitude. As a
byproduct, we obtain a new expression for the related process $\chi\chi\to 2
gluons$, which on the other hand is generally smaller than previously
calculated. There has been a recent claim that evidence for a 3.5 TeV higgsino
annihilating into a $\gamma$ line may already exist from balloon emulsion and
Air Cherenkov Telescope data. We comment on attractive features and problems
with this interpretation.
