"Eventually, all four clusters – each with a mass of at least several hundred trillion times that of the Sun – will merge to form one of the most massive objects in the universe."
Galactic clusters are large clusters of individual galaxies linked together by their collective gravity and the gravity of dark matter.
Gravitational attraction also explains why clusters are suspended in a cloud of gas.
Galaxies typically eject their star gases when stars erupt into supernovae.  Clusters hang on these gases and are one of the largest known objects in the universe.