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 ${10}^6-{10}^9{\mathrm{M}}_{\odot }$. Using a Bayesian formalism supplemented by astrophysical simulations of strong lensing of GWs, we constrain the compact dark matter fraction to $\lesssim 0.4-0.6$ 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.