The "rotation curve" (plot of rotation speed vs. distance from the center) of the Milky Way Galaxy shows that stars in the outer reaches of the Galaxy are orbiting it faster than would be expected from the Galaxy’s visible size. What is the most widely accepted explanation for this effect?
A. Our usual law of gravity must be modified for use over long distances.
B. There is extra, unseen mass within the Galaxy.
C. The measurements of rotation speed are wrong