Why do galaxies form clusters?
The most accepted reason why galaxies form clusters or groups is GRAVITY!!!
The mutual gravity or simple gravity which exists in every object in the universe, also is present inside the galaxies which make them to combine with their neighboring galaxies.
They can even form a cluster with the galaxies which are a million light years far away from them. ( Imagine their gravitational force!!)
A very good example can be :-
One of the nearest galaxy to ours i.e. Andromeda Galaxy moves closer towards us. It comes closer towards us at about 250,000 mile per hour.
This will result in the collision of the two major galaxies which in turn would form a cluster...
A galaxy cluster can have many different types of galaxies consisting in between them.
Our milky way is also a part of a galaxy super cluster which is
Virgo Super cluster.
Here is the representation of the Virgo Super cluster:-