It turns out that the stars further from the center don't take as long as one might think. Observations of this phenomenon led to the development of the theory

It turns out that the stars further from the center don't take as long as one might think. Observations of this phenomenon led to the development of the theory of dark matter. https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve