First, I've spent quite a bit of time over the past couple of days trying to find if this has been addressed anywhere; didn't find it so I'm going to ask.
Dark Matter (DM) is, if I understand it correctly, supposed to be interacting gravitationally with normal matter but not in normal ElectroMagnetic (EM) ways. This is used as a theory to explain such diverse things as why the Universe formed as it has & why there are apparent anomalies in the rotational speeds of galactic disks.
Now this may just be a math issue (as in I don't understand the math) but, there seems to be a requirement that most of the matter in the Universe is both DM AND closely allied to galaxies. Also, it is supposed to be mostly outside the galactic plane, comprising a sphere around the galaxy.
If this is so, it seems most of the galactic mass is not in the central sphere or in the disk, but it IS required to interact gravitationally. It also isn't supposed to interact in any EM way or we'd bee 'seeing' it other than via the lensing effect.
1. How is it that the disk forms at all?
2. Why doesn't the DM distribute the normal matter throughout the sphere as the galaxy forms?
3. Also, why doesn't the gravitational effect seem to work both ways?
If DM is evenly distributed in a shell or sphere, & we have normal matter in a concentrated disk, & the DM is not being kept in place by some sort of motion effect (orbit or repulsion etc.) then over time, the DM should be attracted down to the disk or into the central bulge. (if the bulge has significantly more mass than the disk)
4. For that matter, is it the case that the DM exists outside the disk & central sphere but is not distributed within the normal matter realm? If so, how can that be?
5. Wouldn't that require either EM interaction or physical interaction between normal matter & energy & DM?
I'm a bit puzzled by the explanations I've seen. It seems the DM shell is not required to rotate (or all velocities sum to zero) & it can't interact in any way that would provide a normal effect to the paths of normal matter. Yet it apparently can act upon normal matter in such fashion to produce the observations we have of the visible universe.