The observation that the outer edges of galaxies rotate at roughly the same speed as the inner regions is known as the galaxy rotation problem. This phenomenon is often explained by the concept of dark matter.
When astronomers study the rotational motion of galaxies, they find that the visible matter, such as stars and gas, is not sufficient to account for the observed rotation curves. According to Newtonian mechanics, the outer parts of a rotating object should experience less gravitational pull from the central mass and should therefore rotate at a slower speed than the inner parts. However, this is not what is observed in galaxies.
To explain the consistent rotation speeds, scientists propose the presence of a significant amount of dark matter in galaxies. Dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation, making it invisible to our current detection methods. It is believed to be present throughout the universe and provides additional gravitational pull.
The idea is that the gravitational influence of dark matter helps to hold the galaxies together and affects the rotation curves. The gravitational force from the dark matter dominates in the outer regions, compensating for the lower gravitational pull expected from visible matter alone. This results in a more uniform rotation speed throughout the galaxy.
The nature of dark matter is still a subject of active research, and its exact composition remains unknown. However, its presence is inferred through its gravitational effects on visible matter and the large-scale structure of the universe.
It's important to note that the explanation of dark matter is just one hypothesis among several proposed explanations for the galaxy rotation problem. Alternative theories, such as modified Newtonian dynamics (MOND), have also been proposed to explain the observed rotation curves without invoking dark matter. However, the majority of the scientific community currently supports the idea of dark matter as the most plausible explanation for the observed phenomena.