[frithguild]

if we consider the idea that the motion of galaxies is not determined only by gravity

What else?

[Boogieman]

I would suggest electromagnetism. At the time it was ruled out as a significant factor in the motion of heavenly bodies, we still believed that space was a vacuum, and vacuums cannot conduct electromagnetic forces. We now know that space is mostly filled with very thin, ionized gas, also known as plasma, and plasma is a very good conductor of electromagnetic forces.