Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below
Assuming that the halo is purely made up of dark matter which has only gravitational interaction and nothing else. Therefore, it cannot dissipate energy with time.
Now imagine the system is given a uniform rotation at $t=0$ about the axis shown in the figure.
Wouldn't such a motion cause the dark matter distribution to flatten out and take up the shape of a disc? After all, these dark matter particles will experience a centrifugal force. But this is not observed for the galactic halo. The halo retains its spherical shape even though the galaxy is rotating. What is wrong with my reasoning? Isn't the halo also expected to rotate like the visible matter in the galaxy?