caltech.edu is a pretty reputable source....
Quote:
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"Roughly speaking, a pair of spherical, interpenetrating dark halos interact as if they were single-component systems: the orbital angular momentum of the two halos is transferred to internal degrees of freedom, imparting spin and creating broad tails. ...embedded disks and/or bulges, are not much braked by the tidal forces retarding the dark matter; instead, these components lose orbital angular momentum mostly by interacting with their own surrounding halos, once the latter have been decelerated."
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Not only is the dark matter disrupted in a collision, it sounds like some collisions might even send the involved central galaxies outrunning their dark matter envelopes, or at least temporarily losing their central location, confounding earth observers noticing those galaxies' relative lack of dark matter.