That something else is called dark matter . Astronomers are not sure what the dark matter is made of. Possibilities range from large things like planets, brown dwarfs, white dwarfs, black holes to huge numbers of small things like neutrinos or other particles that have not been seen in our laboratories yet. The nature of dark matter is one of the central problems in astronomy today. Their mutual gravity can draw galaxies together into a cluster that is several millions of light years across.
Some clusters have only a handful of galaxies and are called poor clusters .
Other clusters with hundreds to thousands of galaxies are called rich clusters . The low mass of a poor cluster prevents the cluster from holding onto its members tightly. The poor cluster tends to be a bit more irregular in shape than a rich cluster. There may be more irregular and dwarf ellipticals. The distribution of the galaxies is shown in the figure below.