Gravitational-wave interferometers such as LIGO, Virgo and KAGRA can be used to test the existence of dark matter. While most efforts have focused on finding gravitational waves from heavy dark matter, e.g. primordial black holes mergers, or quasi-monochromatic signals from depleting ultralight boson clouds around black holes, the interferometers can also be used to directly detect dark matter that interacts with various components, e.g. the mirrors or the beam splitter. In this sense, the interferometers act like particle physics experiments: the ultralight dark matter particles, of masses 1e-14 to 1e-11 eV, may interact with baryons or baryon-leptons in the mirrors and cause a quasi-sinusoidal force on them, or alter the values of the fundamental constants in the interferometer components. Even though these signals are not resulting from gravitational waves, both effects will manifest themselves as differential length changes, which can be precisely measured with LIGO, Virgo and KAGRA. We give an overview of the physics of such dark matter interaction signals, and present the results of recent searches for scalar and vector dark matter particles. While no signal has been found, the constraints that come from analyses of LIGO, Virgo and KAGRA data surpass those of other experiments that were designed to specifically search for dark matter (e.g. MICROSCOPE and the Eöt-Wash torsion balance), and represent a bridge between particle physics and gravitational-wave experiments.
