One of the more convincing argument I've heard is the "bullet cluster". Basically it's composed of two clusters of galaxies that recently collided. Since dark m

One of the more convincing argument I've heard is the "bullet cluster". Basically it's composed of two clusters of galaxies that recently collided. Since dark matter and normal matter interact differently (dark matter interacting weakly), you could imagine that the two would have different centers of mass following such a collision. If there's really just modified gravity without any dark matter, the distribution of the regular matter would be sufficient to model the gravitational dynamics but if there is such a thing as dark matter, you'd see that the gravitational effects would be consistent with a center of mass which is displaced from the center of mass for the regular matter. This is, in fact, what you see, suggesting that there really is some type of dark matter. https://en.m.wikipedia.org/wiki/Bullet_Cluster https://en.m.wikipedia.org/wiki/Bullet_Cluster