Newtonian theory predicts that the orbital speed of stars in a galaxy should decrease with distance from the galactic center. This is because the gravitational force exerted by the galaxy's mass decreases with distance. However, observations show that the orbital speeds of stars in spiral galaxies remain relatively constant at large distances from the galactic center. This is known as the flat rotation curve.
General relativity explains this phenomenon by introducing the concept of dark matter. Dark matter is a hypothetical form of matter that does not interact with light, making it invisible. It is believed to make up a significant portion of the mass of galaxies. The presence of dark matter provides the additional gravitational pull necessary to maintain the flat rotation curves observed in spiral galaxies.
Therefore, the flat rotation curves of spiral galaxies are a key characteristic that requires the use of general relativity, as Newtonian theory cannot fully explain them without invoking the existence of dark matter.
