  We consider a model of dark quantum electrodynamics which is coupled with a
visible photon through a kinetic mixing term. After checking of consistency
properties, we compute the $g_w-2$, where $g_w$ is the gyromagnetic factor for
a dark fermion. The $g_w-2$ and $g-2$ of quantum electrodynamics are related by
the kinetic mixing factor. We analyse the $g_w-2$ in terms of the ratio $\kappa
=m_\gamma/m_\chi$ where $m_\gamma$ and $m_\chi$ are the masses of the dark
photon and the dark fermion and we discuss how light and heavy fermions become
very different for $m_\gamma \leq10^{-5}\,$ eV. This analysis is also carried
out using different available data.
