Dark matter is invisible to telescopes, but its gravity was revealed half a century ago by Vera Rubin. Since then, physicists have explored dozens of hypotheses, and one of the most elegant is primordial black holes, formed in the first moments after the Big Bang. No new physics: just objects from the Standard Model and general relativity. But how do you detect something that emits no light? The answer came from an unexpected place — from cold clouds of interstellar dust, heated by the evaporation of these black holes.

The entire Galaxy is littered with microscopic dust grains — silicate and graphite particles drifting in the interstellar medium. Their typical temperature is just a few tens of kelvins, but any extra heat instantly throws them out of equilibrium. That's the key to the method: a black hole with an initial mass of about a mountain (10¹⁵–10¹⁷ g) emits Hawking radiation — a stream of photons and particles whose spectrum depends on mass and, crucially, on spin. The faster the hole spins, the harder and more abundant the radiation. The dust absorbs that energy, and if the heating power exceeds the cooling rate via infrared emission, the grain overheats. In this way, cosmic dust becomes a distributed thermometer spanning the galactic halo. A grain a thousandth of a millimeter across responds to the quantum evaporation of an object with the mass of a mountain — a meeting of the micro and macro worlds on the tip of a cosmic needle.

For the first time, such an analysis systematically accounted for spin and five models of dark matter halo density profiles — from the steep isothermal to the shallow Burkert profile. It turned out that even a moderate spin parameter a* ≈ 0.5 multiplies the photon flux, and in the extreme case a* → 1 the heating is dramatically amplified. The strictest constraints came from the isothermal profile for silicate grains: the fraction of primordial black holes in dark matter (fPBH) cannot exceed 0.01% in the mass range 10¹⁵–10¹⁶ g. Graphite grains are slightly more forgiving — the limit is twice as high. These numbers were obtained through numerical simulations of evaporation spectra and heat transfer, and they significantly fill gaps between classical constraints from lensing and gamma-ray background.

The method turns the entire Galaxy into a detector: trillions of microscopic thermometers sensitive to ghostly heat. Mapping temperature anomalies in dust via infrared surveys could reveal clusters of primordial holes and perhaps point to their origin. This is a bridge between the physics of quantum evaporation and large-scale cosmology — fittingly built on the entropy calculated by Jacob Bekenstein and Stephen Hawking. The next step includes accounting for the actual dust distribution in the disk and 3D modeling of radiation transfer. If primordial black holes truly exist, interstellar dust may become the very signal that reveals their whereabouts and tells the story of the Universe's turbulent youth.