Observational evidence: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to_dark_matter https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...

Observational evidence: https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to_dark_matter https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to... The blue is dark matter. The pink is normal matter. It's thought that the two clumps of blue+pink there passed through each other. But the blues went right through each other, undisturbed, while the normal pink stuff formed a shockwave. >most of the mass in the cluster pair is in the form of two regions of dark matter, which bypassed the gas regions during the collision. This accords with predictions of dark matter as only weakly interacting, other than via the gravitational force.