How come all galaxies don t merge to form a super giant galaxy in galaxy clusters?

As some of you might know, it is very common for galaxies to merge with other galaxies since they are always interacting by gravitational means. That been said, a question arises in me; if they all attract each other by the force of gravity then how come they maintain a certain distance (although not too far) from one another in galaxy clusters? Shouldn t they all merge to form one humongous galaxy?

As a bonus: Does dark matter play a roll here?

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