  We present a search for a new light vector boson, carrier of a "dark force"
between WIMPs, with the KLOE detector at \DA$\Phi$NE. We analyzed $e^+ e^- \to
\mu^+ \mu^- \gamma$ ISR events corresponding to an integrated luminosity of
$239$ pb$^{-1}$ to find evidence for the $e^+ e^- \to U\gamma ,\,\,
U\to\mu^+\mu^-$ process. We found no $U$ vector boson signal and set a 90% CL
upper limit on the ratio of the U boson and photon coupling constants between
1.6$\times10^{-5}$ to 8.6$\times10^{-7}$ in the mass region $520<M_{\rm U}<980$
MeV. A projection of the KLOE sensitivity for the $\mu\mu\gamma$ and
$\pi\pi\gamma$ channels at full statistics and extended muon acceptance is also
presented.
