Constraining compact dark matter with time-varying quasar equivalent widths

One of the possible explanations for dark matter is that of compact dark objects of baryonic origin, such as black holes or even planets. Accumulating evidence, including the discovery of merging stellar mass black holes through gravitational waves, point to a population of such objects making up at least some fraction of dark matter. We revisit a historically heavily used probe, quasar spectra, from the new perspective of time variability and gravitational lensing. From a sample of 777 quasars selected from archival data we identify 19 that show decisive evidence of lensing by compact objects with masses measured in the range $5\times 10^{-5} < M/\mathrm{M}_{\odot} < 2\times 10^{-2}$ with 99\% confidence. This is much lower than what is hoped to be detected by even the most futuristic gravitational wave detectors and analysis strategies, but is crucial for theories of compact dark matter, such as primordial black holes predicted from quantum phase transitions in the early Universe.