Suppose dark matter is normal matter, in an exotic state. Superfluids have some interesting properties in common with "dark matter". Superfluids are quite restricted in how they interact with their surroundings. Because a superfluid is, quantum mechanically, a single entity, all of its interactions are quantized. For example, if the container of a superfluid is spun, the superfluid will not spin until the speed of rotation of the container matches the "critical speed" of the superfluid; at which point the entire bulk of superfluid will immediately start spinning at that speed. Superfluids can be extremely transparent. Consider this: At extremely low temperatures, the quantum wavelength of a particle (particularly (no pun intended) a very light particle, like hydrogen) becomes quite long. There could be a process in which sufficiently cold hydrogen (or helium or something) interstellar or intergalactic gas that is dense enough becomes a superfluid or a supergas or something. Maybe there would be a way to look for this sort of thing. The fact that any transitions would be bulk-quantized should give it some odd properties. Spin, EM absorption, ... Somebody better versed in QM than I would have a better sense for whether transitions into or out of such a state would be observable. Such a cloud of supergas in the vicinity of a supernova could be expected to suddenly change from invisible dark matter to ionized normal matter when the radiation hits. The fact that dark matter outweighs 'normal' matter by 5-to-1 (according to the WMAP 5-year report) suggests that it might have a visible influence, if it does behave in a bulk-quantized fashion. Considering the fact that dark matter seems to be considerably more common than 'normal' matter, maybe I should refer to dark matter as 'normal' and the baryonic matter we're more familiar with as exotic. Suppose a galaxy were surrounded by a ring of dark matter. If (however unlikely) the surrounding ring had a critical rotational speed, then it would likely be a multiple of the rotational rate of the galaxy. A dark matter ring around a galaxy would tend to keep galaxies spinning at constant rates. At least, the outer-most reaches of galaxies would tend to have more stable rotational rates than the rest of the galaxy. Over the long term, this could alter the shape of a galaxy. I'm having trouble visualizing how this might change the appearance, but I'm sure it would. Anyway, just an idea I wanted to toss out there.