For several years, we acknowledged for the existence of very massive galaxy clusters.
Such objects have been shown to be deeply impacted by the nature of Dark Energy, through extensive studies of the halo mass function, and are thus competitive cosmological probes. More recently, we observed clusters merging at very high velocities such as the Bullet Cluster. Inspired by the mass approach, in this talk, I describe the cosmological dependence of the occurrence of such bullet clusters. I first use a statistical approach to study the impact of cosmology on the tail of the relative velocity distribution of dark
matter halos. In order to give further cosmological constraints, I then explain extensively a new model predicting the relative velocity distribution and discuss it, especially in the tail of the distribution.
