Stars in the outer regions of such galaxies appear to be orbiting the centre far too fast. (newscientist.com)
But it is also problematic for the standard dark matter picture, he says, as it's not clear how such a galaxy could form in the first place. (sciencenews.org)
They believe that if such galaxies were common they could have burned off the hydrogen fog that filled the universe then and made it transparent to light. (newatlas.com)