  If dark matter stems from the background of a very light gauge boson, this
gauge boson could exert forces on test masses in gravitational wave detectors,
resulting in displacements with a characteristic frequency set by the gauge
boson mass. We outline a novel search strategy to hunt for such dark matter,
and show that both ground-based and future space-based gravitational wave
detectors have the capability to make a 5$\sigma$ discovery in unexplored
parameter regimes.
