The reason why normal matter forms disks is because of electromagnetic repulsion: electromagnetic atoms naturally resist being in the same place as another atom

The reason why normal matter forms disks is because of electromagnetic repulsion: electromagnetic atoms naturally resist being in the same place as another atom. There becomes a tension between the repulsion to not stack up and gravity to compress things together. Since angular momentum is conserved, the matter will spin and flatten out: ie, a galaxy. If dark mater is a particle, it rarely interacts with anything, including itself; so, it cannot collapse. Most of the particles will be in a constant state of free fall throughout the galaxy. Only if the particle just happens to have a nearly matching velocity would it get trapped in a star or planet. Even then, it would still be wobbling around with nothing to touch.