  The nature of dark matter remains unknown to date; several candidate
particles are being considered in a dynamically changing research landscape.
Scalar field dark matter is a prominent option that is being explored with
precision instruments, such as atomic clocks and optical cavities. Here we
report on the first direct search for scalar field dark matter utilising a
gravitational-wave detector, which operates beyond the quantum shot-noise
limit. We set new upper limits for the coupling constants of scalar field dark
matter as a function of its mass, by excluding the presence of signals that
would be produced through the direct coupling of this dark matter to the
beamsplitter of the GEO$\,$600 interferometer. The new constraints improve upon
bounds from previous direct searches by more than six orders of magnitude, and
are in some cases more stringent than limits obtained in tests of the
equivalence principle by up to four orders of magnitude. Our work demonstrates
that scalar field dark matter can be probed or constrained with direct searches
using gravitational-wave detectors, and highlights the potential of
quantum-enhanced interferometry for dark matter detection.
