It is often stated in these threads that we know that there is not enough baryonic matter to explain the rotation curves.
Do we really know that? The visible part of the Galaxy has many clouds of molecular hydrogen that we can see. Somehow these clouds form in a relatively high radiation environment and become internally cold and dense enough to form stars. Star formation has been going for at least several billion years at the current rate (in the Galaxy).
"Studies of [the Galaxy's] visible disk indicate that most of the mass is in the stars, with only about 2% gas (mostly hydrogen) and about 0.01% dust."
Thus more than 98% of the hydrogen now exists in stars (at least in the visible disk). Where does the hydrogen come from to continue forming stars? The ISM is very sparse.
When galaxies interact, outer parts of arms are sometimes stretched away into long strings that then form stars at very high rates. Doesn't this imply some stable reservoir of hydrogen that need only be "disturbed" to begin forming stars?
We can detect atomic hydrogen out to two or three times the visible radius of galaxies. If there were large quantities of molecular hydrogen at temperatures in the 10 K range shielded by an atmosphere of atom hydrogen, could we detect it? Have we really ruled that out?
If it's so easy to detect H2 and eliminate this possibility, why are estimates based on CO detected? Shouldn't we be skeptical of the arguments that use CO as a tracer? Here there is an assumption that the H2/CO ratio is constant. But what if the outer part of the galaxy is more primordial and contains much less CO. Maybe it's just beyond the ability of our current instruments/techniques to correctly measure the molecular hydrogen content.
If the molecular hydrogen has condensed into planet sized or smaller objects in the outer galaxy (as it has in the solar system), would we have detected it?
Back in 1999, a group of astronomers reported finding large amounts of molecular hydrogen gas when viewing a couple of edge-on spirals. They were only able to detect it directly because it was "warm", IIRC more than 100 K (or higher I don't remember the exact figure). I never saw much follow up on this discovery.
The search for non-baryonic matter is massive, involving the most expensive scientific apparatus ever build and perhaps thousands of people. But I hear very little about the search for large amounts of baryonic matter in the galaxy. There seems to be little interest in that possibility. One cannot help but wonder whether there is little interest simply because it is not "expected" to exist. Scientist generally don't go looking directly for the unexpected. It is usually when they go looking for the expected and get a surprise that science advances.