Here's a breakdown:

* Dark Matter: While we can't directly see it, dark matter is thought to make up the majority of the mass in galaxy clusters. It exerts a strong gravitational pull, acting as the "glue" that binds the galaxies within a cluster.

* Gravity: The combined gravitational force of all the matter in a galaxy cluster, including galaxies, gas, dust, and dark matter, pulls everything together.

* Intercluster Gas: Galaxy clusters are also filled with extremely hot gas, known as intercluster gas. This gas contributes to the overall gravitational pull within the cluster.

It's important to note that galaxy clusters are not static, but are constantly moving and evolving. The gravitational forces within a cluster can cause:

* Collisions and Mergers: Galaxies within a cluster can collide with each other, leading to interactions and mergers.

* Dynamic Movements: The gravitational forces also cause galaxies to move around within the cluster, creating a dynamic and ever-changing structure.

So, while gravity is the primary force that holds galaxy clusters together, the presence of dark matter and intercluster gas are crucial elements in creating and maintaining these massive structures.