> because of dark matter's lack of electromagnetic interaction This is still an assumption, because we don't know for sure that dark matter is only weakly inte

> because of dark matter's lack of electromagnetic interaction This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoking-gun-for-dark-matter/ https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoki... The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle. That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.