Usually (in numerical or real experiments) granular gases are prepared
in a homogeneous situation: uniform-random position of grains,
Gaussian or uniform-random initial velocity with no preferred
direction. It happens that, however the inelasticity of collisions be
strong, the imposed homogeneity is broken after a certain time. The
more accepted scenario is a two time symmetry breaking: at a time
the velocity field becomes unstable to the formation of shear
bands, then at a time 
the density field becomes unstable
toward the formation of high density clusters. After that, everything
may happen.