Because the original assumption about all matter interacting with electromagnetic radiation was proved incorrect for the Milky Way Galaxy, scientists next need to determine whether the assumption still holds true for other galaxies. To do this, scientists need a way to calculate the masses of individual galaxies and galaxy clusters, so they can compare those masses with the luminous matter we actually observe. Astronomers use rotation curves to measure the masses of individual spiral galaxies. The faster the stars orbit about the center of a galaxy, the more mass there is between them and the galactic center. Starting from the hypothesis that other galaxies are made up only of visible matter, astronomers can predict what the galaxy rotation curves would look like if the hypothesis were correct. Sort the galaxy rotation curves according to whether they would support or refute the hypothesis that galaxies are made up only of visible matter.
A) Rotation Curve 4
B) Rotation Curve 3
C) Rotation Curve 1
D) Rotation Curve 2
E) Rotation Curve 6
F) Rotation Curve 5

(If the hypothesis that galaxies are made only of visible matter is correct, then the galaxy rotation curves should show both an increase and a decline in rotational speed. There should be a peak in rotational speed near the region of concentrated starlight in the galaxy's center. There should then be a decline in rotational speed toward the galaxy's edge, where there is a smaller concentration of visible matter. The steady decline in rotational speed should occur because the stars are farther and farther from the galaxy's center. This distance acts to lessen the effect of gravity due to the central mass concentration.

However, actual measurements of galaxy rotation curves show a flattening or steady increase in rotational speeds, even at large distances from the galaxy centers. This result implies that there must be matter within these galaxies that cannot be seen; in other words, dark matter must be present in other galaxies just as it is present in the Milky Way. Therefore, the data scientists collected refutes the hypothesis that galaxies are only composed of visible matter.)