Quick summary
A fast radio burst (FRB) is a millisecond-scale flash from a distant source. As its radio waves cross ionized gas, lower frequencies travel with a slightly larger delay than higher frequencies. Measuring that frequency-dependent sweep gives the integrated number of free electrons along the line of sight, including gas too diffuse to glow brightly.
Dispersion is the key measurement
In a vacuum, radio frequencies travel at the same speed. In a plasma, interactions with free electrons introduce a delay that grows strongly toward lower frequency. Astronomers fit the arrival-time curve and report a dispersion measure, which is proportional to electron density integrated over distance. It is a column total, not a direct three-dimensional map.
Separating the contributors
The measured dispersion includes plasma in the Milky Way, its halo, intergalactic space, the host galaxy and the immediate environment of the source. Galactic electron models estimate the foreground. If the burst is precisely localized and its host redshift measured, researchers can compare the remaining dispersion with cosmic distance.
From electrons to ordinary matter
Most familiar matter in the universe is baryonic, made primarily of protons and neutrons. Some resides in stars and cool gas, while a substantial portion is diffuse and ionized between and around galaxies. Free electrons trace this gas, but converting electron counts to mass requires assumptions about ionization, helium and how matter is distributed.
Why many bursts are better than one
A single sightline passes through an unusual mixture of voids, filaments, galaxy halos and host material. A population of localized FRBs at different distances can estimate an average relation and its scatter. Bursts whose paths pass near known galaxies or clusters can probe how gas is arranged around those structures.
Other propagation clues
Scattering can broaden the pulse, while Faraday rotation reveals magnetic fields weighted by electron density. Repeating bursts allow the environment to be monitored over time. These observables complement dispersion but depend differently on turbulence, magnetic geometry and local plasma.
Reality check
FRBs do not directly detect dark matter. They trace ionized ordinary matter that is hard to see, sometimes called missing baryons. Dark matter does not supply the free electrons responsible for dispersion. Uncertain Milky Way and host contributions can dominate the error for individual bursts.
What improves the cosmic map?
Progress requires more precisely localized bursts, secure host galaxies, redshifts, broad radio-frequency coverage and better foreground models. Transparent treatment of selection effects matters because telescopes do not detect every burst equally through dense or strongly scattered plasma.
