Imagine a bridge with its middle section ripped out — no one expects cars on the other side. That's how astronomers viewed black holes with masses from 50 to 130 Suns: the stars that could create them explode as pair-instability supernovae — the core heats up so much that light spawns particles and antiparticles, pressure collapses, and the star vanishes. But a fresh catalog of gravitational waves brought a surprise: holes with masses of 50–70 Suns were found right in that gap. On top of that, they spin suspiciously slowly for objects born from mergers. The key is the reaction rate of helium with carbon. Fred Hoyle and Hans Bethe proved back in the last century that this microscopic rate governs the fate of giants. New measurements showed the reaction is slightly faster than thought, shifting the gap's edge to 68.5 solar masses, and the forbidden becomes possible. This explains not only the appearance of 'wrong' black holes but also the puzzling scarcity of bright supernovae without hydrogen: stars simply explode less often and more often quietly collapse into black abysses.
Black holes from the forbidden zone spin 5–10 times slower than objects born in mergers. That's the clue: they emerge directly from the collapse of a star — the very one that was supposed to vanish without a trace.