In an underground laboratory in South Dakota sits a tank of liquid xenon. It catches rare events: particles pass through the Earth, strike xenon nuclei, and those recoil like billiard balls after a hit. One such recoil was recorded at an energy of 248 keV, and it turned out to be as lonely as a loud bell strike on a quiet night.
At first, they suspected neutrinos. These particles are almost invisible, but sometimes, flying through xenon, they nudge a nucleus as a whole, and it gets a push. To produce one such push in the right place, you would have to tune a neutrino source to hit the target dead-on. But neutrinos, like a scatterbrained musician, never deliver a single clean strike: if they hit a big bell, they are bound to brush against the little bells around it.
They went through every known neutrino source: from the Sun, from cosmic rays, from the cosmic microwave background. They tried adding neutrinos from dark matter annihilation or from the evaporation of primordial black holes. To get one bell strike, you would have to assume there are more black holes than all the dark matter in the Universe. That is like having more conductors in an orchestra than musicians.
What remains is to admit: elastic neutrino scattering does not explain that lonely strike. Perhaps it was an inelastic encounter, where the particle does not simply bounce off but forces the nucleus to change from within. Or perhaps it is the first ring from a dark matter particle that knows how to hit the target dead-on without touching anything else.
The bell rang once. Now we need to keep listening: if the strikes repeat, we will learn who is ringing.
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.