  If a gamma-ray line is observed in the near future, it will be important to
determine what kind of dark matter (DM) particle could be at its origin. We
investigate the possibility that the gamma-ray line would be induced by a slow
DM particle decay associated to the fact that the DM particle would not be
absolutely neutral. A "millicharge" for the DM particle can be induced in
various ways, in particular from a kinetic mixing interaction or through the
Stueckelberg mechanism. We show that such a scenario could lead in specific
cases to an observable gamma-ray line. This possibility can be considered in a
systematic model-independent way, by writing down the corresponding effective
theory. This allows for a multi-channel analysis, giving in particular upper
bounds on the intensity of the associated gamma-ray line from cosmic rays
emission. Our analysis includes the possibility that in the two-body decay the
photon is accompanied with a neutrino. We show that, given the stringent
constraints which hold on the millicharge of the neutrinos, this is not an
option, except if the DM particle mass lies in the very light KeV-MeV range,
allowing for a possibility of explanation of the recently claimed, yet to be
confirmed, ~3.5KeV X-ray line.
