If you go down a mineshaft, gravity reduces because the pull of gravity is due only to matter between r=0 and the radius R you are at, and force due to all matter outside that radius R where you are integrates to zero. Assume, perfectly spherical, uniform matter density distribution. But now extend the matter distribution to infinity while you remain at R. Classical theory says you experience no change. But when in the limit you are in an infinite, spherical, homogegous distribution of matter, why should you feel a pull in any particular direction? This is relevant to understanding whether "Dark Matter" really solves anything as regards the anomalous galactic rotation.