Galaxies are not distributed homogeneously in the Universe. Some
regions in the Universe have a galaxy density (i.e. the number of
galaxies per Mpc
) significantly lower than the mean, and some a
significantly higher density. The main reason for this is that gravity
amplifies small inhomogeneities: if a region is slightly denser than
its surroundings (for example a group of galaxies), it will tend to
attract more matter than the surrounding regions, and hence become even
more massive (turn into a galaxy cluster, say). This is the essence of
hierarchical structure formation: small structures merge together to
make more massive one.
The galaxies huddle together in groups of a few galaxies - such as the MW and M31 in the Local Group - and some are in clusters with hundreds of galaxies. The groups and clusters themselves are clustered in super clusters (i.e. clusters of clusters). But there are no clusters of super clusters: on sufficiently large scales the Universe is not a fractal but becomes homogeneous.