Constraints on compact dark matter from the non-observation of gravitational-wave strong lensing

We use the non-observation of strong lensing of gravitational waves (GWs) in the first three observation runs of LIGO-Virgo detectors to constrain the fraction of dark matter in the form of compact objects in the mass range 10⁶-10⁹~M_. Using a Bayesian formalism supplemented by astrophysical simulations of strong lensing of GWs, we constrain the compact dark matter fraction to 0.4-0.6 with currently available data and show that they may get significantly tighter in the future. We find that multiple lensing -- i.e., GWs getting deflected by multiple compact objects on their way to us -- is possible. By ignoring this, we underestimate the constraints by a few percent.