According to the theory of general relativity, a galaxy must have a certain amount of mass to be held together by gravity. However, scientists don’t see enough visible mass in many galaxies in the universe, yet gravity keeps such galaxies intact. How’s this even possible?

This is where the concept of dark matter comes into play. Scientists believe that galaxies have invisible mass in the form of matter that doesn’t interact with light. The gravity holding these galaxies exists because of this invisible mass.