  The absence of a breakthrough in directly observing dark matter (DM) through
prominent large-scale detectors motivates the development of novel tabletop
experiments probing more exotic regions of the parameter space. If DM contains
ultralight bosonic particles, they would behave as a classical wave and could
manifest through an oscillating force on baryonic matter that is coherent over
$\sim 10^6$ periods. Our Helium ultraLIght dark matter Optomechanical Sensor
(HeLIOS) uses the high-$Q$ acoustic modes of superfluid helium-4 to resonantly
amplify this signal. A superconducting re-entrant microwave cavity enables
sensitive optomechanical readout ultimately limited by thermal motion at
millikelvin temperatures. Pressurizing the helium allows for the unique
possibility of tuning the mechanical frequency to effectively broaden the DM
detection bandwidth. We demonstrate the working principle of our prototype
HeLIOS detector and show that future generations of HeLIOS could explore
unconstrained parameter space for both scalar and vector ultralight DM after
just an hour of integration time.
