How are galaxy clusters held together?

Galaxy clusters are primarily held together by gravitational forces among the galaxies contained within them. The mass of the individual galaxies, along with the mass of the intergalactic gas and dark matter that surrounds and permeates the cluster, generates a gravitational pull that keeps the members of the cluster bound together. This gravitational binding is crucial for maintaining the structural integrity of a galaxy cluster over cosmic timescales.

While dark matter also plays a significant role in the overall mass and structure of the universe, it does so indirectly by contributing to the total gravitational force within the cluster. However, the direct mechanism by which galaxies are held together is through their mutual gravitational attraction, resulting from their masses.

Electromagnetic radiation does not contribute to the binding of galaxy clusters; instead, it is the force of gravity that dominates and keeps the galaxies from flying apart. Supernova explosions may release vast amounts of energy and can affect local star formation, but they do not act as a binding force for the overall structure of a galaxy cluster. Instead, they are events that may occur within galaxies but do not influence the gravitational cohesiveness of the cluster as a whole.