[image error]Despite their origin not yet being fully understood, FRBs have served as a tool to detect ‘missing baryonic matter’.

In 1997 the ratio of heavy hydrogen (containing an extra neutron) to ordinary hydrogen was used to calculate how much baryonic matter there should be in the universe; baryonic matter being, to us, ordinary matter; made of protons and neutrons. The answer was about 5%, the rest being dark matter and dark energy. But counting up everything that could be observed only added up to about half of that – the rest is referred to as the ‘missing baryon problem’. A further confirmation of that 5% was, in 2001, deduced from tiny fluctuations in the CMB (Cosmic Microwave Background).

The leading theory for the missing matter has always been that it exists as a plasma, a warm-hot intergalactic medium (WHIM).

Now, baryonic plasma will just marginally slow down longer wavelengths of radio frequency more than shorter wavelengths. In the last couple of years it has become possible to pinpoint the source galaxy of FRBs, and thus we can measure the dispersion (difference in arrival time of short and long wavelengths) in the FRB signal, relate that to the distance to that galaxy, and deduce the quantity of WHIM that it has travelled through. Sure enough, the results are indicating the existence of that missing 2.5%.

Further, as more of these directional measurements are made, it should be possible to map the density of WHIM, which is suspected to exist as a filamentary network connecting galaxies – the ‘cosmic web’.