  We study an experimental search of a GeV scale vector boson at
BES-III/BEPCII. It is responsible for mediating a new U(1)$_d$ interaction, as
recently exploited in the context of weakly interacting massive particle dark
matter. At low energy $e^+ e^-$ colliders this dark state can be efficiently
probed. We discuss the direct productions of this light vector $U$ boson and
the decay of this state with BES-III data and its foreseen larger data. In
particular, we show that Higgs' strahlung in the dark sector can lead to
multilepton signatures, which probe the physics range for kinetic mixing
parameter $\epsilon \sim 10^{-4} -10^{-3}$ over a large portion of the
parameter space.
