  We perform a global QCD analysis of high energy scattering data within the
JAM Monte Carlo framework, including a coupling to a dark photon that augments
the standard model electroweak coupling via kinetic mixing with the hypercharge
$B$ boson. Including the most recent measurement of the anomalous magnetic
moment of the muon as a constraint, we find a significant reduction in the
combined $\chi^2$, favoring the inclusion of a dark photon, with a statistical
significance in excess of 8$\sigma$. With respect to the experimental data, the
improvements in the theoretical predictions are spread across a wide range of
$x$ and $Q^2$, with the largest improvement corresponding to neutral current
data from HERA, while the best fit yields a value of $g-2$ which significantly
reduces the disagreement with the latest experimental determination. The best
fit yields a dark photon mass in the range 4.2--6.2 GeV and a mixing parameter
of order 0.1.
