Why don’t galaxies revolve around each other?

Answer

If celestial bodies move around each other, it is because of the mutual gravitational attraction they exert on each other. If they were to start from a standstill, or were to move towards each other initially, they would just fall on top of each other. If they have a component of motion that does not point to each other, they accelerate as they approach, but they miss each other; once they pass each other, they slow down again, until finally they move closer again, missing each other again. If there are only those two bodies, they will continue to move around each other in an elliptical orbit.

The moon revolves around the sun with the earth. But they also feel each other, and on top of that common motion around the sun, they move together around the center of mass between their two centers of mass: that is still inside the earth, but about 5,000 km from the center toward the moon. In general, all celestial bodies feel each other, but a certain hierarchy occurs: the earth does feel the attraction of the other planets, but they are so small in relation to the sun and usually so far away that the approximation that the earth mainly feels the sun , is quite accurate. If the sun moves around the center of the galaxy, it’s because that’s the galaxy’s center of mass, and it’s a good approximation to say that all those stars together describe a symmetrical mass distribution around the center.

If two galaxies were isolated from everything else, they would also orbit each other elliptically; a very large job, of course, in which one sees hardly any displacement in a lifetime. That is roughly what the Large Magellanic Cloud, a close companion to our galaxy, does to our galaxy. But in general, galaxies feel the pull of many galaxies at the same time, with similar masses at similar distances from each other. Their motion is then the sum of all those individual accelerations, and a closed orbit is usually not. The resulting movement can therefore be quite erratic. In large clusters of galaxies you sometimes have thousands of galaxies, all of which more or less attract each other. For example, these systems describe complicated orbits through the cluster. But they can’t get out, because galaxies that hit the edge are pulled more or less in the same direction by all the others and fall back.