Three questions, all related. Firstly, I'm wondering what sort of modifications to Newtonian gravity were tried to explain the flatness of various galaxy rotation curves. (References, and especially a review, would be much appreciated. I haven't been able to find anything appropriate.) I assume having the gravitational "constant," G, increase with distance, was tried and found wanting, which led to Milgrom's notion of increased acceleration in low field strength environments (MOND). So, secondly, why is it that having the gravitational "constant" instead vary with distance (increasing, then decreasing and perhaps even becoming negative, i.e., giving rise to repulsive gravity over very long distances) doesn't work? Thirdly, are there any computer programs available on-line that would allow one easily to play with such changes to G and see what effect it would have on the rotation curves?