You've all seen the set-up. Bore a hole thru the center of the earth, and jump in. As you approach the center, the only portion of the earth's mass that exerts an accelerative gravitational influence/geometry on you is that portion between you and the center, the "external" portions balance/cancel out. So now enlarge the scale to galaxies. They shouldn't rotate as they do, which was one of the the earliest indicators of the existence of dark matter. All of the illustrations/maps I've seen (e.g. - Bullet Cluster) of the assumed/calculated locations and distributions of dark matter show a homogeneous "cloud" surrounding the galaxies, or even offset as in the case of the Bullet Cluster. Granted that the scales are vastly different, the principles remain the same. And those two illustrations are apparently contradictory. The dark matter halo beyond the edge of a galaxy should exert no influence on the bodies closer to the galactic center. The only solution I see to this conundrum (misunderstanding on my part?) is that the density of dark matter increases as it approaches the galactic center, much as the density of regular matter does - I've never seen anything that addresses this. Does this sound like a reasonable assumption, and can anyone point me to articles that address it?