Astronomers have made measurements on an “ultra-diffuse” galaxy – it is the same size as our galaxy but only contains about 1% of the number of stars in our milky way. Indeed the star density is so diffuse that it is relatively transparent and other galaxies can be observed through it.
Measurements of the movements of 10 globular star clusters within the galaxy show that those globular clusters are moving much slower than expected – in fact at a speed consistent with there being no dark matter in the galaxy – the movement of the clusters being entirely explained by the mass of the stars in the galaxy.
This unexpected result is not readily explained. It brings existing theories of galaxy formation into question, and also puts question marks around the behaviour or existence of dark matter as it is currently understood.
Update 4th July 2018: A new research paper, not yet peer reviewed, has challenged the above results, claiming that the galaxy is much closer than stated previously. I will update again when the peer review process clarifies the situation.
Coincidentally, published today in the journal Nature, is a study showing that the Equivalence principle, the idea that Gravitational Mass is equal to Inertial Mass, holds, even with very high mass objects like neutron stars and white dwarfs. This in turn rules out several theories in which gravity varies from Relativity in extremes, and therefore makes the theoretical existence of Dark Matter seem even more likely.
Update 25th January 2019: Another diffuse galaxy has now been found and the velocity-dispersion results in both confirmed by much more sophisticated telescope equipment. It now seems probable therefore that these two galaxies are indeed examples of galaxies lacking dark matter. Ironically this strengthens the dark matter theory, because it rules out modified gravity theories as an alternative.