We study the consequences of an extension of the standard model
containing an invisible extra gauge group under which the SM particles
are neutral. We show that effective operators, generated by an
high-energy physics connecting the new gauge sector to the Standard
Model, can open a new portal and give rise to a viable dark matter
candidate. Its annihilations produce clean visible signals through a
gamma-ray line. This would be a smoking gun signature of such models
observable by actual experiments.