Does dark matter form orbits around stars ? Why would dark matter and "regular" matter, say, a dust belt not mingle and stick together? Just with black holes, with d.matter we are positing new objects with complex properties instead of simply admitting imperfection in gravity. Since it is difficult to show how gravity is mediated (as opposed to showing its effects, which are clear)...to me, Occham's razor is best applied by modifying gravity rules locally instead of inventing new substances. Of course, it is inelegant to make exceptions to gravity, which works well. On the other hand, gravity is so strange as to be suspicious: something leaves a mass at lightspeed, collides with other objects: the effect of this collision is to suck the object against the direction of the impact, and it does not weaken from use, and penetrates any type of element ? While gravity generally applies, it is so strange that one should be willing to think about exceptions.