  The existence of a new force beyond the Standard Model is compelling because
it could explain several striking astrophysical observations which fail
standard interpretations. We searched for the light vector mediator of this
dark force, the $\mathrm{U}$ boson, with the KLOE detector at the DA$\Phi$NE
$\mathrm{e}^{+}\mathrm{e}^{-}$ collider. Using an integrated luminosity of 1.54
fb$^{-1}$, we studied the process $\mathrm{e}^{+}\mathrm{e}^{-} \to
\mathrm{U}\gamma$, with $\mathrm{U} \to \mathrm{e}^{+}\mathrm{e}^{-}$, using
radiative-return to search for a resonant peak in the dielectron invariant-mass
distribution. We did not find evidence for a signal, and set a 90\%~CL upper
limit on the mixing strength between the Standard Model photon and the dark
photon, $\varepsilon^2$, at $10^{-6}$--$10^{-4}$ in the 5--520~MeV/c$^2$ mass
range.
