rosbourne noted this on page two of this thread but he used a bigger word than I will. But that is a nice word. I like it
. "Barycenter", pure poetry
Any two objects, be they galaxies or dust motes, if they are gravitationally bound, will orbit the center of their masses.
One never orbits around the other. Consequently if the masses are very different as in the Earth-Sun combination the center of the masses will always be very close to the center of the sun and it will appear to a casual observer that we go around the sun.
Now lets take two larger solid (for clarity) masses of equal weight
If they are gravitationally bound each will travel an orbit around the center of the masses which will be between them.
Do you see how this makes it possible to have a galactic cluster or a local group with nothing in the middle
Each galaxy is orbiting around the center of mass of the cluster or group of which it is a part . There need be no mass in the center