The universe is full of mysteries, not least of which is that there appears to be quite a lot of it "missing". That sounds ominous, but it's just that observations don't quite match up to the theory, while the theory otherwise makes sense. Scientific types understandably don't like that kind of discrepancy.
In this instance, the detection of elusive "Fast Radio Bursts" allowed observers to make some calculations and account for the formerly "missing" matter - kind of A Big Deal.
The gravity in the universe is far greater than can be accounted for by what we observe. Astronomers believe that most of this is accounted for by dark energy, which makes up 70 percent of the universe, and dark matter, which makes up 25 percent of the universe.
The remaining five percent is ordinary matter, and it's what everything we see is made of.
But all the observed ordinary matter, from all the stars and galaxies and planets and nebulas, only adds up to about half of what should be there if this model of the universe is correct.