Current constraints from bullet clusters on the cross-section of particulate dark matter are 18 orders of magnitude larger than that required to distinguish between plausible particle-physics dark matter candidates (for example from supersymmetric extensions to the standard model). In order to investigate a physically interesting régime of dark matter cross-section, and provide smaller error bars, many more individual bullet clusters are required. However collisions between two massive progenitors are rare and ultimately the total observable number of such systems may be inadequate.
In , a method for using every individual infalling substructure in every cluster has been proposed. For each piece of infalling substructure, a local vector from the dark matter peak (identified using weak lensing analysis) and the baryonic mass peak (from X-rays) – dubbed ‘bulleticity’ – can be defined
In Figure 35 a result from full hydrodynamical simulations of dark and baryonic matter within clusters in shown.  have used these simulations to show that the measurement of a net bulleticity consistent with the cold dark matter used in the simulations is possible.
Finally, a Fisher matrix calculation has shown that, under the assumption that systematic effects can be controlled, Euclid could use such a technique to constrain the relative particulate cross-sections to .
The raw bulleticity measurement would constrain the relative cross-sections of the baryon-baryon interaction and the dark matter-dark matter interaction. However, since we know the baryonic cross-section relatively well, we can infer the dark matter-dark matter cross-section. The dark matter-dark matter interaction probed by Euclid using this technique will be complementary to the interactions constrained by direct detection and accelerator experiments where the primary constraints will be on the dark matter-baryon interaction.
Living Rev. Relativity 16, (2013), 6
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