What primarily dictates the gravitational forces holding galaxy clusters together?

Dark matter

While galaxy clusters contain visible components like galaxies and hot gas (ICM), the gravitational evidence overwhelmingly indicates that the majority of the mass required to keep these structures gravitationally bound, especially against the high velocities of their constituent galaxies, comes from dark matter. Dark matter is an invisible substance that does not interact with light but possesses mass, contributing the scaffolding upon which the cluster is built. The observed mass discrepancy between the luminous matter (stars and gas) and the gravitational mass inferred from dynamics or lensing is what highlights the dominance of this unseen component in maintaining cluster integrity.