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We perform numerical simulations of the merging galaxy cluster 1E 0657-56(the Bullet Cluster), including the effects of elastic dark matter scattering.In a similar manner to the stripping of gas by ram pressure, dark matterself-interactions would transfer momentum between the two galaxy cluster darkmatter haloes, causing them to lag behind the collisionless galaxies. Theabsence of an observed separation between the dark matter and stellarcomponents in the Bullet Cluster has been used to place upper limits on thecross-section for dark matter scattering. We emphasise the importance ofanalysing simulations in an observationally-motivated manner, finding that theway in which the positions of the various components are measured can have alarger impact on derived constraints on dark matter's self-interactioncross-section than reasonable changes to the initial conditions for the merger.In particular, we find that the methods used in previous studies to place someof the tightest constraints on this cross-section do not reflect what is doneobservationally, and overstate the Bullet Cluster's ability to constrain theparticle properties of dark matter. We introduce the first simulations of theBullet Cluster including both self-interacting dark matter and gas. We findthat as the gas is stripped it introduces radially-dependent asymmetries intothe stellar and dark matter distributions. As the techniques used to determinethe positions of the dark matter and galaxies are sensitive to different radialscales, these asymmetries can lead to erroneously measured offsets between darkmatter and galaxies even when they are spatially coincident.

What does the Bullet Cluster tell us about self-interacting dark matter?