  We study the phenomenology of a $U_X(1)$ extension of the Standard Model
where the SM particles are not charged under the new abelian group. The
Green-Schwarz mechanism insures that the model is anomaly free. The erstwhile
invisible dark gauge field $X$, even if produced with difficulty at the LHC has
however a clear signature in gamma-ray telescopes. We investigate what BSM
scale (which can be interpreted as a low-energy string scale) would be
reachable by the FERMI/GLAST telescope after
  5 years of running and show that a 1 TeV scale can be testable, which is
highly competitive with the LHC.
