Clusters of galaxies form through gravitational binding rather than random encounters, as the latter would take longer than the universe's age. These clusters are either systems where member galaxies became gravitationally bound simultaneously or represent condensations from pregalaxian material, leading to the formation of galaxies within them. Resources such as the Universe Review and the CSEP website provide detailed overviews and classifications of galaxy clusters.
PREREQUISITESAstronomers, astrophysicists, and students of cosmology seeking to understand the formation and classification of galaxy clusters.
It is highly improbable that clusters of galaxies have been built up by chance encounters of galaxies in the general field. It just takes too long (longer than the age of the universe) to complete the process. Therefore, either that clusters are systems whose member galaxies became gravitationally bound at more or less the same time, or that the clusters represent condensations from pregalaxian material and that subcondensations within them became galaxies.