The stars on the outer edges of galaxies go around faster than they should be. How is dark matter meant to account for this? if you just add more mass wont that just give it a greater gravitational force? So we would still expect the outer stars of galaxies to be going slower than the ones near the center. Whats the flaw in my reasoning?
Yes, more mass would mean a greater gravitational pull. A greater gravitational pull, in turn, means that an object would have to be going faster at a particular distance if it is moving in a circle. Similar to an object tied to a string and going in a circle: the faster it's moving, the larger the required force (in this case tension).