That's the better way to phrase the question. Galaxies start off with everything (visible and dark matter alike) going in all directions. Visible matter experiences friction with other visible matter; the upshot is (and, sorry, this is by no means obvious, but it does work) that the matter segregates over time into a low-angular-momentum, flat, corotating disk and some high-angular-momentum ejecta. Dark matter, absent friction, never gets modified this way. The Earth happens to be in a galaxy where this has taken place, and we happen to be in the disk.
Dama
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That's the better way to phrase the question. Galaxies start off with everything (visible and dark matter alike) going in all directions. Visible matter experiences friction with other visible matter; the upshot is (and, sorry, this is by no means obvious, but it does work) that the matter segregates over time into a low-angular-momentum, flat, corotating disk and some high-angular-momentum ejecta. Dark matter, absent friction, never gets modified this way. The Earth happens to be in a galaxy where this has taken place, and we happen to be in the disk.
Acleron
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My thanks to ben m and Dancing David. It's much clearer now.
I certainly didn't realise that the residual angular velocity of visible matter was because of ejection of high angular velocity components, I sort of thought that opposing angular velocities cancelled each other out and left what we see now.
I certainly didn't realise that the residual angular velocity of visible matter was because of ejection of high angular velocity components, I sort of thought that opposing angular velocities cancelled each other out and left what we see now.