The phenomenon you are referring to is known as "galactic rotation curves." In galaxies, including spiral galaxies like the Milky Way, stars and other visible matter are concentrated near the center in a region called the galactic bulge. Surrounding the bulge is a disk of stars and gas, extending out to the outer edges of the galaxy.
According to Newton's laws of motion, objects that are farther away from the center of a rotating system should move at slower speeds than those closer to the center. In other words, if galaxies were composed only of visible matter, we would expect the stars in the outer regions to orbit more slowly than those near the galactic center.
However, observations have shown that the rotation curves of galaxies do not behave as expected based on the visible matter alone. Instead, the rotation curves remain relatively flat as a function of distance from the galactic center. This implies that the stars in the outer edges of galaxies are moving at unexpectedly high speeds, much higher than what would be predicted by the gravitational influence of the visible matter.
The most widely accepted explanation for this phenomenon is the presence of dark matter. Dark matter is a form of matter that does not emit, absorb, or reflect electromagnetic radiation, making it invisible to direct detection. It is thought to be distributed throughout galaxies, including their outer edges, and exerts a gravitational pull on visible matter.
The gravitational influence of dark matter can explain the observed rotation curves of galaxies. The additional mass provided by the dark matter effectively increases the gravitational pull and allows stars in the outer regions to move at higher speeds, balancing the gravitational attraction from the visible matter in the galactic bulge.
The exact nature of dark matter remains a topic of ongoing research and investigation. Various theories and experiments are being pursued to understand its composition and properties better. However, the evidence for the presence of dark matter in galaxies, as revealed by the consistent rotation curves, remains one of the key pieces of evidence supporting its existence.