The evidence is mounting, as reported by Space.com, that there is something seriously wrong with the current, dark matter, explanation of galactic rotation. To date, at least 19 galaxies have been discovered that appear to be missing dark matter altogether. What this means, in practice, is that these galaxies are rotating so slowly that no additional matter, beyond their complement of visible baryons, is required to explain their gravitational coherence. Only fast-spinning galaxies need the gravitational attraction of extra matter to hold them together; our traditional understanding of gravity is enough to keep the slow-spinning ones from flying apart.
However, the current LCDM theory postulates that dark matter exists all by itself, as an independent substance, and should be present in large quantities even in galaxies that do not need it. In other words, just because a galaxy is rotating slowly enough to remain intact without dark matter, does not imply that its dark matter is not present – only that it is not necessary to hold it together.
To explain the latest observations what we really need is a phenomenon that only exists when it is necessary. Dark matter would only be a candidate if it made sense to say that it comes into existence as-needed and vanishes where it is unnecessary. Very strange stuff indeed.
Thankfully, there is a perfectly reasonable and elegant explanation for galactic rotation that does not require us to either reject Newton or accept the existence of dark matter. Spiral Galaxy Rotation in chapter 4 of The Substance of Spacetime discusses this.