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.
interesting. I am not a cosmolgist at all, but certainly we know that super massive black holes at the galactic centers have bipolar astrophysical jets, which implies axial rotation. It makes sense to me that this may generate a powerful electomagnetic field perpendicular to the axis of rotation.
I have an honest question.
Can or is the Speed of Light slowed down while traveling through different medium?