Recent observations indicate that about half of the dwarf satellite galaxies around M31 orbit in a thin plane approximately aligned with the Milky Way. It has been argued that this observation along with several other features can be explained if these dwarf satellite galaxies originated as tidal dwarf galaxies formed during an ancient merger event. However if dark matter is collisionless then tidal dwarf galaxies should be free of dark matter - a condition that is difficult to reconcile with

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... servations indicating that dwarf satellite galaxies are dark matter dominated. We argue that dissipative dark matter candidates, such as mirror dark matter, offer a simple solution to this puzzle.