What I've heard is that galaxies are spinning weirdly. They are spinning on the outer regions too fast. Also that the velocity of the outer stuff is 10 times greater than the gravitational force of our galaxy itself. So we say that there is this dark matter that we can't see at any wave lengths that makes up for 90% of a galaxy's mass. This accounts for the galaxy staying in one peice but how does this account for the outer regions moving as fast as they are? If dark matter is effected by gravity then it would clump into the middle of the galaxy like everything else and you would still get a very similar ratio of gravity throughout the galaxy that you would if it didn't exist. Could it be that dark matter doesn't clump into the middle and is evenly distributed? Would that even help explain the speed of the outer stuff either? Plus this seems wrong if it is effected by gravity unless there was another force doing some repelling so that the density of the darkmatter was very low (if that makes sense). I hope my rambling makes sense, and I know some of what I've said is assuming a little bit, but... if you need me to clarify, I'd be happy to.