  In dark matter (DM) models, the production of a gamma line (or of a
"box-shaped" gamma-ray spectrum) from DM annihilation proceeds in general from
a loop diagram involving a heavy charged particle. If the charged particle in
the loop carries also a color charge, this leads inevitably to DM annihilation
to gluons, with a naturally larger rate. We consider a scenario in which DM
candidates annihilate dominantly into gluon pairs, and determine (as far as
possible, model-independent) constraints from a variety of observables: a) the
dark matter relic density, b) the production of anti-protons, c) DM direct
detection and d) gluon-gluon fusion processes at LHC. Among other things, we
show that this scenario together with the recent claim for a possible gamma
line from the Galactic center in the Fermi-LAT data, leads to a relic abundance
of DM that may be naturally close to the cosmological observations.
