Nature Astronomy, Published online: 01 June 2026; doi:10.1038/s41550-026-02856-z The mass spectrum of binary black-hole mergers has been expected to show a ‘mass gap’ above 45 solar masses, consistent with the physics of pair-instability supernovae. An extensive catalogue of gravitational-wave detections reveals a high-spin population above this threshold that probably results from repeated black-hole mergers in dense star clusters, and fits with the existence of the mass gap. The inferred bound