Triplet of 'ghostly' galaxies hints at what dark matter is and isn't

Also see [1], a follow up post by Stacy McGaugh, a MOND proponent, who is quoted in the Quanta article. He does not agree with the evidence as presented in that article.

[1] https://tritonstation.com/2026/10/15/ghostly-galaxies-in-the...

> If two typical galaxies met in a head-on collision, the dark matter would experience very little friction, or drag, and would keep on rocketing by. But ordinary matter, which interacts using all four fundamental forces, would get all tangled up.

Isn't a galaxy mostly empty space? I thought I read before that when galaxies collide there's actually very little physical contact between the matter. Stars mostly pass through affected predominantly only by change in surrounding gravity. Is this not the case?

https://www.quantamagazine.org/triplet-of-ghostly-galaxies-h...

[dead]