In the late 1990s, cosmologists made a prediction about how much ordinary matter there should be in the universe. About 5%, they estimated, should be regular stuff with the rest a mixture of dark matter and dark energy. But when cosmologists counted up everything they could see or measure at the time, they came up short. By a lot.
The sum of all the ordinary matter that cosmologists measured only added up to about half of the 5% what was supposed to be in the universe.
It took the discovery of a new celestial phenomenon and entirely new telescope technology, but earlier this year, our team finally found the missing matter [doi.org].
But when radio waves pass through matter, they are briefly slowed down. The longer the wavelength, the more a radio wave "feels" the matter. Think of it like wind resistance. A bigger car feels more wind resistance than a smaller car.
The "wind resistance" effect on radio waves is incredibly small, but space is big. By the time an FRB ["Fast Radio Burst"] has traveled millions or billions of light-years to reach Earth, dispersion has slowed the longer wavelengths so much that they arrive nearly a second later than the shorter wavelengths.
We were overcome by both amazement and reassurance the moment we saw the data fall right on the curve predicted by the 5% estimate [doi.org]. We had detected the missing baryons in full, solving this cosmological riddle and putting to rest two decades of searching.
The initial results are based on six data points, FRBs; the researchers will continue to look for others.