Answer: A.unexpected orbital velocities of stars in galaxies
Explanation:
Dark Matter makes up about 80% of matter in the universe. This matter does not emit light and hence, it is called dark matter as it cannot be observed directly.
The effect of the presence of the dark matter is unexpected orbital velocities of stars in galaxies. At the center, the orbital velocities are greater than those at the edge. Had there been no dark matter, the galaxy would have flown apart.