The dynamical evidence suggests that significant amounts of dark matter exist in spiral galaxies, but the nature of the dark matter is very uncertain. In the current models the dark matter halo is seen to contribute an increasing amount to the rotational velocity of a galaxy as distance from the nucleus increases:
This can be seen in the lower panel of the first figure in the above link. The contribution from the halo increases while the contribution from the disk decreases.
An interesting possibility is that a significant portion of the dark matter could be normal baryonic dark matter. This possibility was put forward in 1994 by Pfenniger, Combes, & Martinet:
Pfenniger et al propose that the dark matter is primarily cold molecular gas such as H2 in a rotationally supported thick disk. The gas is thought to be in fractal subunits of about 30 AU and containing 10-3 solar masses. If dark matter is in the form of cold gas then several problems might be explained:
1. Cold gas could provide a reservoir to feed the relatively constant star formation rate in spiral galaxies.
2. The ratio of dark mass to stellar mass decreases along the Hubble sequence from Sd to Sa type galaxies, but the dark matter to HI mass remains constant. Since one scenario is that Sd galaxies involve into Sa galaxies, this would be expected if the dark matter is being converted into stars along with atomic hydrogen.
3. This might also explain the disk-halo conspiracy: http://nedwww.ipac.caltech.edu/level5/Bosma/frames.html
One difficulty is that molecular hydrogen is very difficult to detect. However, a number of recent studies indicate that there may be a significant amount of molecular gas and dust in the halo:
1. Velentijn find that NGC 891 has enough molecular hydrogen to explain the dark matter at least in the optical disk: http://adsabs.harvard.edu/cgi-bin/np...e5c03c80a07381
2. Kalberla et al find evidence for significant amounts of H2 in the galactic halo with the flattened distribution predicted by Pfenniger et al:
3. Walker&Wardle find evidence for significant amounts of Jupiter mass cool gas clouds in the galactic halo from Extreme Scattering Events:
4. Trewhella et al find evidence for a significant component of cold dust in galaxies and conclude that: “But perhaps the most important implication of this result is the suggestion that the far-infared emission is associated with cold halo material that could, in turn make up some or all of the missing mass in galaxies. It is also significant to note that they find the cold dust is extended 10 x more radially than vertically which is consistent with the proposal of Pfenniger et al that the baryonic dark matter is in a flattened thick disk.
5. Richter, Sembach, & Howk find evidence for H2 in a dense filament in the Milky Way Halo. They note that it is unknown whether structures such as this filament are related to the structures proposed by Pfenniger et al.
So in total there is an interesting possibility that a significant – if not all – of the dark matter in the Milky Way could be composed of normal baryonic matter.