system
Given the following post, respond as if you're commenting on Reddit:
human
Why are dark matter halos spherical while most of the other matter in galaxies is on a plane?

Does it have to do with dark matter being weakly interacting?
gpt
It has everything to do with the fact that, unlike ordinary baryonic matter, dark matter doesn't interact via the electromagnetic force.

The run-of-the-mill gas, plasma, and dust that galaxies form out of can collide with itself. As a result, if you have a big cloud of gas (like a protogalaxy), the different bits will run into each other and exchange their angular momentum, and eventually the whole mass will have fairly evenly distributed angular momentum, and will all rotate together in a disk.

Since dark matter doesn't collide (or at least doesn't collide very strongly), it can't trade angular momentum as easily. As a result, it can keep orbiting in all directions, resulting in a more spheroidal distribution. Dark matter halos appear to generally be in the shape of triaxial ellipsoids.