  For the milli-charged Dirac dark matter in the Stueckelberg Z' model, we
discuss the contributions from the vector couplings of the dark matter with the
neutral gauge bosons to the spin-independent scattering cross section in the
direct detections. We also compute the effective coupling between the fermionic
dark matter particle and the standard model Higgs boson generated through a
triangular loop of Z and/or Z' bosons which may contribute to the
spin-independent scattering cross section at the quantum level. We show that
the latter contribution is consistent with the most recent experimental limits
from CDMS II and XENON100. In the case that the dark matter particle carries a
milli-charge of order O(10^{-3}e), we found that it would lose all its kinetic
energy by colliding with nucleons in the atmosphere before reaching the
detector. Even though we use the Stueckelberg Z' model for illustration, the
results we obtain are rather general and applicable to other Z-Z' portal-type
hidden-sector models as well.
