  A dark sector containing a Higgs field and a dark photon coupled to visible
photons via a small kinetic mixing is considered. The mass of the dark gauge
boson becomes rescaled by the parameter involved in the kinetic mixing. We also
calculate the total annihilation cross section for scalar Higgs interacting
with an external field and show that exhibits Sommerfeld enhancement. The model
can be extended to a $SU(3)\times SU(2)\times U(1)$ dark sector. The mass of
the dark $Z$ boson increases due to the kinetic mixing.
