Gas experiences electromagnetic interactions and this is enough to effectively keep the gas of the two clusters from continuing to move past each other, with the result that the gas that was initially moving along with the clusters gets gridlocked in the middle. The dark matter, on the other hand, interacts very little—both with the gas and, as the Bullet Cluster demonstrates, with itself.
Ok, that is very cool. If I understand this correctly, the ordinary matter acts like two people walking into each other -- they collide, as expected. The dark matter colliding with ordinary matter passes right through -- weird, but expected with what we know so far. Dark matter colliding with dark matter -- I would have expected it to act like people walking into each other, but it passes through. Weird as fuck.
It makes me wonder if there are multiple forms of dark matter, each of them non-interactive with the others. If the two galaxies formed around different types of dark matter, their ordinary matter would collide, but their dark matter would pass through each other.
Which seems more absurd? Dark matter doesn't even interact with itself, or there are multiple types of matter we cannot directly observe rather than just one? Dunno.
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