The discrepancy between what was expected and what has been observed has grown over the years, and we're straining harder and harder to fill the gap.
Jeremiah P. OstrikerThere are reasons, increasing in number and quality, to believe that the masses of ordinary galaxies may have been underestimated by a factor of 10 or more.
Jeremiah P. OstrikerIf you take a galaxy and try to make it bigger, it becomes a cluster of galaxies, not a galaxy. If you try to make it smaller than that, it seems to blow itself apart.
Jeremiah P. Ostriker