(In)visible Z' and dark matter
  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 loops of heavy chiral fermions charged
under both gauge groups and connecting the new gauge sector to the Standard
Model, can 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.
