There have been observations where the observed additional matter 'lagged behind' colliding galaxies, because dark matter only interacts gravitationally. If gra

There have been observations where the observed additional matter 'lagged behind' colliding galaxies, because dark matter only interacts gravitationally. If gravity itself were to behave differently, and the additional mass would always come from the observed matter, the dark matter should have been observed right where the colliding galaxies were, but since it didn't, it's a good indicator for the actual existence of gravitationally-interacting dark matter. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster