  We propose an inclusive search for dark photons $A'$ at the LHCb experiment
based on both prompt and displaced di-muon resonances. Because the couplings of
the dark photon are inherited from the photon via kinetic mixing, the dark
photon $A' \to \mu^+ \mu^-$ rate can be directly inferred from the off-shell
photon $\gamma^* \to \mu^+ \mu^-$ rate, making this a fully data-driven search.
For Run 3 of the LHC, we estimate that LHCb will have sensitivity to large
regions of the unexplored dark-photon parameter space, especially in the
210-520 MeV and 10-40 GeV mass ranges. This search leverages the excellent
invariant-mass and vertex resolution of LHCb, along with its unique
particle-identification and real-time data-analysis capabilities.
