How rare is the Bullet Cluster (in a $\Lambda$CDM universe)?
  The Bullet Cluster (1E0657-56) is well-known as providing visual evidence of
dark matter but it is potentially incompatible with the standard $\Lambda$CDM
cosmology due to the high relative velocity of the two colliding clusters.
Previous studies have focussed on the probability of such a high relative
velocity amongst selected candidate systems. This notion of `probability' is
however difficult to interpret and can lead to paradoxical results. Instead, we
consider the expected number of Bullet-like systems on the sky up to a
specified redshift, which allows for direct comparison with observations. Using
a Hubble volume N-body simulation with high resolution we investigate how the
number of such systems depends on the masses of the halo pairs, their
separation, and collisional angle. This enables us to extract an approximate
formula for the expected number of halo-halo collisions given specific
collisional parameters. We use extreme value statistics to analyse the tail of
the pairwise velocity distribution and demonstrate that it is fatter than the
previously assumed Gaussian form. We estimate that the number of dark matter
halo pairs as or more extreme than 1E0657-56 in mass, separation and relative
velocity is $1.3^{+2.0}_{-0.6}$ up to redshift $z=0.3$. However requiring the
halos to have collided and passed through each other as is observed decreases
this number to only 0.1. The discovery of more such systems would thus indeed
present a challenge to the standard cosmology.
