At nine in the morning, an underground detector in Italy catches a faint flash. By evening, the number of such flashes is almost half. It's not a glitch: it's "dark weather."

This is how dark matter reveals itself — invisible stuff that doesn't emit light but pulls on stars. It's constantly streaming through Earth. But as a particle flies through rock, it can hit an atomic nucleus and lose energy. In the morning, the detector faces into the galactic wind of these particles head-on: they have to travel just a few hundred kilometers. By night, Earth has rotated, and the particles must bore through the entire planet—12 700 kilometers. Many get stuck, and the stream weakens. Earth's rotation daily changes the intensity of this invisible rain.

The detector itself registers particles differently. They can knock an electron out of an argon atom, leaving an electrical signal. That signal points to an interaction via electromagnetism (the electric force). But the journey through Earth depends on something else — collisions with nuclei. By comparing morning and evening signals, scientists separate these two types of interaction. In the Standard Model, the rulebook for familiar particles, there's no place for dark matter. One hypothesis introduces a dark photon, a mediator particle similar to an ordinary photon but from the dark sector. In that model, the interaction strength with electrons and with nuclei is tightly linked. Any deviation from that link would hint at more complex physics.

Numerical simulation has shown that the shape of the daily signal is very sensitive to nuclear scattering. An analysis of DarkSide-50 data, conducted without considering time of day, has already set a strict limit: the nuclear interaction must be weaker than a certain threshold. Future detectors, running for years and placed around the globe, will be able to read "dark weather" as meteorologists read clouds.

Every day, Earth runs its own experiment. The detector under Gran Sasso mountain diligently records morning and evening signals. And one day, this daily pulse will tell us not only about the "weather" underground but also about how the universe is built.