If our galaxy's dark matter halo is so large and diffuse, why is the ordinary matter in it so much more localised and compactly located?

I just read that our galaxy's dark matter halo is estimated to be 1.5m ly across, compared to the visible galaxy's 100k ly across, needed to explain stellar rotation curves.

Why would this be? By which I mean, why would "ordinary" matter have become so localised and comparatively dense in a much smaller space, than the DM whose attraction gave rise to it?

Also, if DM is 4x the amount, but spread across that size it must also be very diffuse, in which case a very large part of the DM would seem to have (almost) zero effect on galactic rotation as it's (probably approximately?) uniform and outside the visible galaxy, hence like being inside a uniform shell, there should be little or no net gravitational effect either.

• Shouldn't be related to the fact that ordinary matter can 'stick"? At least in part? – Alchimista Mar 12 '18 at 10:05