Hypothesis
LIGO-Virgo O2 data could detect, or at least meaningfully constrain, primordial black hole dark matter through mergers pairing a subsolar component with a primary above about 2 solar masses.
What failed
Four candidates passed a signal-to-noise threshold of 8 and a false alarm rate of 2 per year, but none was statistically significant; assuming a null, the 90% upper limits on the primordial black hole dark-matter fraction stayed above 1 for every mass function and both early and late binary formation prescriptions, so no part of the parameter space was excluded.
Broken assumption
Existing detector sensitivity suffices to constrain primordial black holes in the QCD-transition mass window.
Why
The authors state plainly that O2 sensitivity is insufficient to place a meaningful constraint under the assumed mass model.
From the published paper: Phukon et al. Physical review. D/Physical review. D., 2026. ↗CC BY