The Wilkinson Microwave Anisotropy Probe (WMAP) was launched in 2001 and allowed scientists to finally determine the makeup of the Universe. Surprisingly, the Universe appeared to be 4.6% ordinary atoms (baryonic matter), 24% dark matter, and 71.4% dark energy. Among the 4.6% ordinary matter, however, about a third was missing or unaccounted for.
This missing matter was theorized to exist in the vast expanse between galaxies as Warm-Hot Intergalactic Medium (WHIM) but until now had evaded detection.
With the aid of exotic and powerful Fast Radio Bursts (FRB) from distant galaxies, however, a team of scientists was finally able to detect this heretofore missing ordinary matter.
While the exact sources of these FRBs are not known (possibly created by accelerated matter falling into rapidly spinning neutron stars or black holes), a team of scientists led by J. Xavier Prochaska of UC Santa Cruz using the Australian Square Kilometre Array Pathfinder was able to calculate the intergalactic baryon density precisely by measuring the distance, frequencies, and relative timing of FRBs from five separate galaxies.
They relied on the fact that high-frequency high-energy waves would transit the intergalactic medium faster than their low-frequency lower-energy counterparts. By precisely calculating the lag times and factoring in the distances to the respective galaxies, the scientists were able to determine the baryon density along the line of sight for each of the FRBs. The average came out to about 1 baryon per cubic meter, which fits with predictions derived from the previous WMAP surveys.
While this is a good start, further FRB observations will hopefully yield more detailed calculations.