  We search for single-photon events in 53 fb^-1 of e+e- collision data
collected with the BaBar detector at the PEP-II B-factory. We look for events
with a single high-energy photon and a large missing momentum and energy,
consistent with production of a spin-1 particle A' through the process
e+e->gamma A', A'->invisible. Such particles, referred to as "dark photons",
are motivated by theories applying a U(1) gauge symmetry to dark matter. We
find no evidence for such processes and set 90% confidence level upper limits
on the coupling strength of A' to e+e- in the mass range m_A'<=8 GeV. In
particular, our limits exclude the values of the A' coupling suggested by the
dark-photon interpretation of the muon (g-2) anomaly, as well as a broad range
of parameters for the dark-sector models.
