The original post: /r/askscience by /u/superealsuccessor on 2024-10-08 03:31:59.

We have determined that galaxies are held together by gravity that is much stronger than the sum of their mass would suggest. We use dark matter to explain this. Is this model accounting for the energy within the galaxy? Since energy and mass are the same thing sort of (e = mc2,) this energy should exert gravity too. Yes, energy would have a much much weaker gravitational pull than mass, but think of how much energy is in a whole galaxy. Think of all of the angular momentum and motion of all the millions of stars and black holes in the whole system. Would the net of all of this gravity help hold the galaxy together, with the barycenter at Sagittarius A*?

Not trying to debunk dark matter or anything, just trying to learn about physics!