The enormous distances between stars are not spaces entirely devoid of matter. The interstellar spaces are filled with ‘dust’: very low density matter. This minuscule amount of matter, spread over almost infinite distances, acts like a curtain obscuring the stars that lie behind. If it were not for this material we would see no dark patches in the sky at night: the sky would be entirely covered with stars.

The two parts in boldface play what roles in the argument above?

A. The first is a suggestion that the author wishes to dispute. The second is hypothesis that the author wishes to explain.
B. The first is the main point the author wishes to make. The second is a hypothetical result of accepting that point.
C. The first is a fact that the author thinks is important in explaining a certain phenomenon. The second is a result that the author would expect if that fact were not true.
D. The first is a speculation that the author wishes to justify. The second is a consequence that would result if that speculation is not true.
E. The first is an established fact that the author wishes to explain. The second is a consequence of accepting this fact.

the answer is C: 1st is fact, 2nd is: result
my question is why not D ?? I felt like the sentence The enormous distances between stars are not spaces entirely devoid of matter is a speculation/theory that the argument verifies.

any explanation please??

Thanks