In a deep mine sits a tank of liquid xenon. It catches rare impacts: particles from space fly into the tank, hit xenon nuclei, and those nuclei recoil. One day the detector registered a single strong kick, as if someone had struck a big bell.
The scientists wondered: could this be a neutrino? Neutrinos are tiny particles that pass through everything, but sometimes they nudge a whole nucleus. To produce one strong kick, a neutrino would have had to hit exactly one nucleus. But a neutrino can't strike with precision: if it hits hard, it inevitably grazes many other nuclei more weakly.
What remains is the mystery: who struck the bell? Maybe it was a dark matter particle, one that can hit its target dead-on. For now, nobody knows. The detector keeps listening.
Obtaining a single event from primordial black holes would require their fraction in dark matter to be $f_{PBH}$≥95, which means there would have to be more black holes than all the dark matter in the Universe. That is physically impossible.