This animation shows an artist's representation of the huge collision in the Bullet Cluster.
From here on Earth, we are able to see the Bullet Cluster across our line of sight, and hence the process of merger can be seen clearly. In Abell 576, where two galaxy clusters are merging head on, in our line of sight, it is harder to disentangle.
During the collision the hot gas (shown in red) in each cluster is slowed and distorted by a drag force, similar to air resistance. A bullet-shaped cloud of gas forms in one of the clusters.