“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 groupings of individual galaxies bound together by their collective gravities and the gravity of dark matter.
The gravitational attraction also explains why clusters are suspended in a cloud of gas.
Galaxies will typically expel their stellar gases when stars erupt into supernovas.
Clusters hang onto these gases and are some of the largest known objects in the universe.