Well, the particles also have no way to radiate away energy, so the dark matter particles just whisk around each other under the gravitational force.

You can check out something like this on your computer

http://http.developer.nvidia.com/GPUGems3/gpugems3_ch31.html

To see what pure 1/r^2 interactions look like. You can download the simulations from here:

http://developer.nvidia.com/cuda/cuda-toolkit-40