A NewScientist article carries some more details. Apparently this was part of a search for “dwarf” galaxies, which are predicted to be present in the universe at much greater frequencies than so far observed. But this galaxy is no dwarf at all – it just has no stars. Other “dark matter” galaxies have been found in the past, but all had at least a few stars, though in one case (dwarf galaxy I Zwicky 18) all the observed stars were less than 500,000 years old, a tiny fraction of the normal billions of years.
It’s possible this observation is just a coincidence caused by two hydrogen clouds passing one another at high speed, rather than a single rotating mass of hydrogen. Nevertheless, we know the universe is mostly made of dark matter – it’s got to be out there somewhere, and this may be the first real evidence where.