What if dark matter gamma-ray lines come with gluon lines?

In dark matter (DM) models, the production of a γ line (or of a ``box-shaped'' γ-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 antiprotons, (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 γ 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.