How do they (CERN) know that dark matter is a particle, but not some unknown property of gravity at large scale?

Source:

https://www.quora.com/How-do-they-CERN-know-that-dark-matter-is-a-particle-but-not-some-unknown-property-of-gravity-at-large-scale-What-are-the-problems-with-the-idea-that-dark-matter-could-be-just-gravity-Why-are-most-people-so-sure

"Q: What are the problems with the idea that dark matter could be just gravity?

A: Because this" (image)