Although It Hasn't Been Observed (Yet), Why Do Astronomers Think

Although it hasn't been observed (yet) , why do astronomers think that the Milky Way may contain "dark" material?

A) Theory predicts that there should be more black holes than we've so far detected.
B) The Galaxy's gravity seems too big for the mass astronomers can see.
C) Due to its large size, the light from stars on the other side of the Milky Way has not reached us yet.
D) Astronomers have detected neutrinos from unseen sources.