The fact that the dark matter halos surrounding colliding galactic clusters simply pass through each other without interacting has a simple explanation. If they are superfluid bodies, wouldn't be reasonable to simply assume that their relative speed is less than the super-fluid critical velocity. Then, there wouldn't be a significant momentum transfer between bodies until their collision speed exceeds that velocity.

This might also explain DM free galaxies, if upon colliding, the collision speed is greater than critical, heating effects might change a cold dark matter halo into hot dark matter that cannot be gravitationally bound.


Dark matter doesn't have to be a superfluid not to interact with itself. We know dark matter doesn't interact via the strong or EM forces because if it did we would have seen it by now. So at most it interacts via the weak force and the weak force is ... well ... weak.

Having said that, it has been been suggested that dark matter forms a superfluid phase under the right circumstances. As far as I know this was first suggested by Berezhiani and Khoury in Theory of Dark Matter Superfluidity. However the motivation for this is not allowing dark matter to pass through itself in galaxy cluster collisions. Indeed in this theory the superfluid only forms in galaxies where the dark matter is relatively dense - the superfluid is not formed in lower density regions like galaxy clusters. The motivation is that when the superfluid forms it allows collective excitations that slightly modify the gravitational interaction in a way that matches the MOND theory.

For completeness, we should note this is not considered a mainstream theory by most physcists.


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