It has been hypothesized that much of the matter in the universe is "d

It has been hypothesized that much of the matter in the universe is "dark"; i.e., unseen. Studies have shown that galaxies in many galaxy clusters are moving faster with respect to one another than they would if visible stars constituted all their mass. The studies suggest that the galaxies are moving under the gravitational influence of unseen mass in considerable quantity.

Which of the following is an assumption underlying the passage above?

(A) Measurements of the speed of moving galaxies are extremely unreliable.
(B) The workings of gravitational forces ar.e not particularly well understood.
(C) The aggregate mass of visible stars in the galaxies mentioned above can be estimated with some confidence.
(D) The general composition of unseen matter in the universe has been determined.
(E) Without exception, the galaxies mentioned above move toward one another.

C is the answer.

The crux of the argument is that the galaxies are moving at a faster speed than they wound under only gravitational pull visible stars. So there must be some other reason. That reason is the gravitation pull of the dark matter.
We have to find the assumption.

A it actually weakens the argument. If measurements are unreliable then the falls.

B it also weakens the argument. If workings are not well known then the hypothesis is also not correct.

C correct. If the effect of visible star is known with certainty then we can safely validate the hypothesis about dark matter.

D This choice does not help us. It gives us redundant information about composition.

E This also superfluous information with no tie to the conclusion.

Hope it helps