If you imagine that dark matter and visible matter both start out basically smoothly distributed in a big cloud, they'll both tend to collapse inwards. The mat

If you imagine that dark matter and visible matter both start out basically smoothly distributed in a big cloud, they'll both tend to collapse inwards. The matter forms a rotating disk because the particles of matter interact through non-gravitational means; so ones that get close can exchange (and cancel!) momentum. But if dark matter only interacts gravitationally, it won't do this; the particles will just fall through the center of the future galaxy, 'passing through' each other (and the matter). Gravitational drag presumably adds some rotation to the dark matter over time; I don't know. But that's how dark matter and regular matter could separate. >So does dark matter interact with itself outside of gravity? There are different ideas about what dark matter could be. Some have it interacting through other forces (WIMPs interacted through the weak force, for example), some are purely gravitational. There's always talk of 'dark matter candidates', theoretical or otherwise particles that could plausibly be dark matter.