The universe is like a vast invisible scaffold. We don't see it directly, but we know it's there: galaxies and clusters hold together thanks to it. This scaffold is dark matter. The most popular version says it is "cold," made of heavy, slow-moving particles. But this picture has a problem: it predicts too many small clumps of matter around large galaxies, and observations don't confirm that.

One possible solution is warm dark matter. Its particles are lighter and faster, smoothing out small irregularities in the early universe, like a gentle breeze flattening ripples in sand. But for this idea to work, the particles must have a specific mass. And that's where gamma-ray bursts come in — the most powerful explosions in space. They are so bright that they are visible from enormous distances, nearly from the edge of the observable universe.

The authors took data from 118 gamma-ray bursts recorded by the Swift satellite over two decades and compared their frequency at different distances with how many stars were being born at those times. The logic is simple: if warm dark matter is too light, it will prevent the formation of the first stars, and thus there will be fewer gamma-ray bursts. This approach allowed them to set a lower limit on the particle mass: at least 1.3 keV (a kiloelectronvolt is about 500,000 times lighter than a proton). And if you assume that the burst rate strictly follows the star formation rate, the constraint becomes even tighter—3.4 keV.

This is not just an abstract number. It narrows the search, ruling out a whole class of dark matter models. Gamma-ray bursts turned out to be a surprisingly precise tool for "weighing" the invisible. And the more we learn about the connection between these explosions and star birth, the more accurate our scales will be.