A flashlight beam cuts through fog. The denser the fog, the more the light scatters and the later it reaches your eye. A similar principle works with fast radio bursts — mysterious, super-powerful pulses from distant galaxies. As their radio waves travel through space, they interact with free electrons — particles stripped from atoms. This interaction delays the signal. The more electrons in the path, the greater the delay. By measuring this delay (called the dispersion measure), astronomers can 'count' all the invisible matter between galaxies.

This new work turns that principle into a precise scientific tool. Using an approach similar to analyzing galaxy spectra (spectroscopy), the authors built a model linking radio burst delays to the distribution of galaxies on the sky. The key result: free electrons turn out to be an almost perfect 'mirror' of all matter. Their clumps and rarefactions trace the cosmic web with high accuracy. The correlation coefficient is close to one. This means that a map of electrons is essentially a ready-made map of dark and visible matter.

The practical value is enormous. Future sky surveys, such as LSST and Euclid, will be able to use radio bursts to calibrate their weak lensing measurements, where the unknown gas distribution is a major nuisance. In the first years of the new telescopes, data will become so abundant that this method will become standard. It's like being able to see not only the stars but also the very air between them, and understanding how that air moves and is stirred by galaxies.