Dark Matter Suspected to Exist in the Universe Because

Multiple Choice

Dark matter suspected to exist in the universe because:

A) the night sky is dark between stars
B) the orbital period of stars in the outer parts of a galaxy is greater than the orbital period of stars near the galactic center
C) the orbital period of stars in the outer parts of a galaxy is less than the orbital period of stars near the galactic center
D) the orbital period of stars in the outer parts of a galaxy is about the same as the orbital period of stars near the galactic center
E) all galaxies have about the same mass