Answer:
Unexpected orbital velocities of stars around the centers of galaxies led astronomers to predict the existence of dark matter.
Step-by-step explanation:
Dark matter forms most of the universe. It is called dark matter because it is not visible. Only its presence can be detected. Scientists are not sure about the composition of the dark matter. The orbital velocities of stars in the galaxies were different than what was expected which led astronomers to predict the existence of dark matter. The presence of dark matter can explain these unexpected orbital velocities.