This is the picture and article that made Dark Matter "click" for me, as a total layman: https://www.nasa.gov/mission_pages/chandra/news/08-111.html

With lensing we can "see" that dark matter (or whatever) passed through each other on a collision, while some "normal" matter actually collided and stopped in the middle (or slowed down).

Now, maybe "modified gravity" still makes sense somehow, but how come this modified gravity is following the previous path of a cluster in space like that?

This is as opposed to "The galaxies rotate too fast" argument, where the obvious problem is "Maybe it's rotating exactly as it should and you just don't know how fast it should rotate?"

This is a comment entirely about what's inside my head, not about the actual astronomical merit of any theory.