How do we know there is dark matter in other galaxies? a. it occasionally lum inesces b. flat rotation curves in spiral galaxies c. matter rotates slower the farther from the center of a spiral galaxy d. it blocks our view in elliptical galaxies

b. flat rotation curves in spiral galaxies

Explanation:

Dark matter accounts for majority of the matter in the universe (about 85%). It is considered to be composed of subatomic particles which are yet to be discovered. Dark matter cannot be observed because it does not emit light that can be detected. Only its gravitational effect is visible.

Astronomers when calculated the mass of the visible matter and tried to account for the observed speed of stars in the galaxy, there was a mismatch. To understand what caused this mismatch, astronomers gave name to the invisible matter as the Dark Matter.  

The expected velocity of stars should decrease away from the near the center of the galaxy but a constant velocity of stars was observed that is a relatively flat rotation curve was obtained.  

Thus, flat rotation curves in spiral galaxies in an evidence of Dark Matter.