I still don't quite understand what the stars in galaxies without SBHs revolve around. I mean, the stars must orbit something. If the Sun suddenly disappeared, the planets wouldn't just continue in their orbits as there's nothing to orbit. E.g. M33 is thought to not have an SBH in its center. If the barycenter is in the very center of the galaxy wouldn't that require that all stars revolving around it have the same mass relative to their distance to each other, forming a gargantuan multiple-star-system? And how would such a system be stable over the course of thousands of years? Perhaps we are wrong and there is an SBH or another massive star at the center of each galaxy?