  Hints in the Fermi data for a 130 GeV gamma line from the galactic center
have ignited interest in potential gamma line signatures of dark matter.
Explanations of this line based on dark matter annihilation face a parametric
tension since they often rely on large enhancements of loop-suppressed cross
sections. In this paper, we pursue an alternative possibility that dark matter
gamma lines could arise from "semi-annihilation" among multiple dark sector
states. The semi-annihilation reaction with a single final state photon is
typically enhanced relative to ordinary annihilation into photon pairs.
Semi-annihilation allows for a wide range of dark matter masses compared to the
fixed mass value required by annihilation, opening the possibility to explain
potential dark matter signatures at higher energies. The most striking
prediction of semi-annihilation is the presence of multiple gamma lines, with
as many as order N^3 lines possible for N dark sector states, allowing for dark
sector spectroscopy. A smoking gun signature arises in the simplest case of
degenerate dark matter, where a strong semi-annihilation line at 130 GeV would
be accompanied by a weaker annihilation line at 173 GeV. As a proof of
principle, we construct two explicit models of dark matter semi-annihilation,
one based on non-Abelian vector dark matter and the other based on retrofitting
Rayleigh dark matter.
