The observation that the outer edges of galaxies rotate at roughly the same speed as the inner regions is known as "galactic rotation curves." This phenomenon has led to the development of the concept of dark matter.
According to Newtonian physics, the gravitational force exerted by the visible matter in a galaxy should decrease with increasing distance from the galactic center. As a result, the orbital velocities of stars and gas in the outer regions of a galaxy should decrease. However, measurements of galactic rotation curves have consistently shown that the orbital velocities remain relatively constant with distance from the center.
To explain this discrepancy, scientists have proposed the existence of dark matter. Dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation, making it difficult to detect directly. It is thought to permeate galaxies and the universe on a large scale, exerting gravitational effects on visible matter.
The presence of dark matter can account for the observed flat rotation curves of galaxies. The additional gravitational pull from dark matter compensates for the decrease in gravitational force from visible matter, allowing stars and gas in the outer regions to orbit at higher speeds than expected. This implies that the majority of a galaxy's mass is in the form of invisible dark matter.
While the nature of dark matter remains unknown, its existence is supported by various lines of evidence, including galactic rotation curves, gravitational lensing, and the large-scale structure of the universe. Scientists continue to study dark matter and its properties to gain a better understanding of its role in the dynamics and evolution of galaxies.