Correct Response: C. The rotational velocity of galaxies can be estimated by calculating how gravity should be affecting the observable mass in a particular galaxy. In the 1970s, the calculated value for galactic rotational velocity predicted from the visible mass of stars and gas in a galaxy was shown to be different from the observed rotational velocity of the galaxy. Astronomers concluded that dark matter in galaxies was the missing mass responsible for the discrepancy between the calculated and observed rotational velocities. Redshift of distant galaxies (A) provides evidence of an expanding universe, but not dark matter. The high energy emitted by quasars (B) is produced by supermassive black holes at the center of massive galaxies and has not provided evidence of dark matter. The size of galaxy superclusters (D) results from the gravitational attraction that draws galaxy clusters together and has not provided evidence for the existence of dark matter.
