The Bullet Cluster is typically touted as the biggest evidence that dark matter is right and MOND is wrong: https://en.wikipedia.org/wiki/Bullet_Cluster#Signifi

The Bullet Cluster is typically touted as the biggest evidence that dark matter is right and MOND is wrong: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to_dark_matter https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to... Basically, the center of mass in the cluster, and the center of the matter we can see, are way far apart from one another, in a way that ends up being super clear to observe. We can infer the center of mass by the surveying the gravitational lensing we observe, and we can observe the center of the baryonic matter by looking at the x-ray sources. The offset in distance here ends up being pretty clear evidence that most of the mass exists as stuff we can't see (in any wavelength, only via the lensing.)