Astronomers have known for years that the ordinary matter we see every day -- made up of protons, electrons, and neutrons -- can only make up a small fraction of the mass-energy density needed to explain the large-scale structure of the universe. This ordinary, or "baryonic" matter, makes up around 4% of the critical amount. Another 23% or so is "dark matter", which isn't made of protons, electrons or neutrons, but does exert gravitational forces like baryonic matter; and the remaining 73% or so is the very mysterious "dark energy", which acts sort of like anti-gravity.
When most scientists see the phrase "missing matter", they think of the "dark matter" portion of the universe -- the 23%.
But this new result gives us information on a portion of the 4%, the ordinary baryonic matter. We think it should make up 4% of the critical density because of the relative abundances of hydrogen, helium, and lithium which were produced soon after the Big Bang
This new study looked at radio waves from an event in a very distant galaxy. Those radio waves had to traverse a very long distance to reach us. As they flew through space, IF that space had even very thin traces of gas, waves of some frequencies would travel just a bit faster than others. That dispersion in frequency acts to spread out the arrival of the radio waves by the time they reach the Earth. The astronomers mentioned here observed a small spread in arrival times and used to to figure out how much gas the waves must have encountered in between the galaxies. The result: just the right amount of gas to account for all those hidden baryons.
So, yes, this study found missing baryons. It did not produce any direct measurements of dark matter or dark energy. On the other hand, if we can pinpoint other fast radio bursts in the future and study their host galaxies, we may learn something about those other entities, too.