At https://tallbloke.wordpress.com/2016/09/22/researchers-find-new-natural-… … we learn that in the late 1970s astronomers Vera Rubin and Albert Bosma independently found that spiral galaxies rotate at a nearly constant speed. The implication was that stars and gases inside a galaxy do not decrease with radius as one might expect from Newton's law of the distribution of 'visible' matter. It remains approximately constant.
Subsequently, such rotation issues have been attributed to invisible dark matter surrounding galaxies and provided gravitational attraction. Now, a team from the Case Western Reserve University has found a new relationship in spiral and irregular galaxies – the acceleration observed in rotation curves correlates with the gravitational acceleration expected from visible mass alone (completely upending previous theories). The looked at 153 galaxies in their study in order to confirm their findings. This seems to suggest we can do away with the necessity of invoking dark matter. No doubt it is not the end of the story but it will exercise minds.
See also http://arxiv.org/abs/1609.05917v1
The findings have been compared to a fundamental law and a comparison made between the distance separating planets in the solar system and the distances (on a larger scale) between galaxies in the universe.