Local gravity refers to the gravitational force exerted by the visible matter within a galaxy, such as stars and gas. Dark matter, on the other hand, is a theoretical form of matter that does not interact with light and is believed to make up a large portion of a galaxy's mass.
Local gravity plays a significant role in determining the rotation speed of a galaxy. The distribution of visible matter, which produces the local gravitational force, affects the rotation curve of a galaxy. This curve shows how the rotational velocity changes as you move away from the center of the galaxy.
It is possible for local gravity to override the effects of dark matter in galactic rotation. In some cases, the distribution of visible matter can be enough to explain the observed rotation curve of a galaxy without the need for dark matter. However, this is not always the case and dark matter is still believed to play a significant role in galactic rotation.
There are several lines of evidence that support the existence of dark matter in galaxies. These include observations of galactic rotation curves, gravitational lensing, and the cosmic microwave background radiation. Additionally, simulations of galaxy formation and evolution also require the presence of dark matter to match observed structures.
Scientists use a variety of methods to study the effects of local gravity and dark matter in galactic rotation. These include observations with telescopes and satellites, computer simulations, and theoretical models. By combining these approaches, scientists can gain a better understanding of the complex interactions between local gravity and dark matter in galactic rotation.