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

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... m 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.