What keeps galactic clusters together?
Gravity (the attraction force between 2 or more objects with mass)
The universe is expanding at an accelerating rate making the distance between all celestial bodied longer. The gravitational force holding our galaxy together is greater than the force pushing us apart making us stay in our galaxy. The same thing applies to our galactic cluster. Our cluster is massive enough for gravity to overcome the dark energy pushing us apart, keeping our galactic cluster together.