  The detection of gravitational waves from merging binaries has ushered in the
era of gravitational wave interferometer astronomy. Besides these strong,
transient, calamitous events, much weaker signals can be detected if the
oscillations are nearly monochromatic and "continuous", that is, coherent over
a long time. In this work we show that ultra-light dark matter of spin two,
owing to its universal coupling $\alpha$ to Standard Model fields, generates a
signal that is akin to but distinct from a continuous gravitational wave. We
show that this signal could be detected with current and planned gravitational
wave interferometers. In the event of a null detection, current facilities
could constrain the coupling to be below $\alpha\sim10^{-7}$ for frequencies of
tens of Hz, corresponding to dark matter masses around the $10^{-13}$ eV mark.
Future facilities could further lower these upper limits and extend them to
smaller masses down to $10^{-18}$ eV. These limits would be the most stringent
bounds on the spin-2 Yukawa fifth force strength, parametrised by $\alpha$, in
the frequency ranges accessible by gravitational wave interferometers. The
implementation of this type of searches for gravitational wave interferometers
would therefore further our grasp of both dark matter and gravity.
