Scientists know the universe is expanding because of the a. blue shift of distant galaxies. b. red shift of distant galaxies. c. blue shift of the brightest galaxies. d. red shift of the brightest galaxies.

The correct answer is
B. red shift of distant galaxies.

Basically, the farther it is the more space there is to stretch and expand. This means that the further away the galaxy is the further away from us it is moving. Scientists observe this through observing red shifts which and all their observations point out to the idea that the universe is constantly expanding.