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Some of them might have been born in the early universe instead of from dying stars.","posed_since":"2016","precise":"Determine the fraction of the LIGO-Virgo-KAGRA binary black hole merger rate contributed by primordial black holes, or an upper bound on it, using mass, spin and redshift distributions. A sub-solar-mass black hole ($M < 1\\,M_{\\mathrm{sun}}$) in a merger would strongly favor primordial origin, since stellar collapse cannot make black holes below about $1\\,M_{\\mathrm{sun}}$ (the remaining alternative being neutron stars converted to black holes by captured dark matter); mergers at redshift $z > 30$ seen by third-generation detectors would also point to primordial origin.","problem_ref":"de6054bbd1a6195f2e031d87c4a4de1915b78c639a4f90d48d3e74df18b83db4","references":"","settled_by":"Detection or exclusion of sub-solar-mass compact binary mergers, plus redshift-resolved merger rates from Einstein Telescope or Cosmic Explorer.","status_note":"Mroz et al. (2024) bound compact halo objects of $1.8 \\times 10^{-4} \\text{ to } 6.3\\,M_{\\mathrm{sun}}$ below 1 percent of dark matter; the O4 candidate S251112cm (2025), found by the sub-solar-mass search, has low significance (false-alarm rate about 1 per 6 yr), and no confident sub-solar-mass merger exists.","title":"Do any observed black-hole mergers involve primordial black 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