Bosma (1978, 1981b) and Carignan et al. (1990) found a trend for the gas distribution to have the same shape as DM distribution. This correlation between gas and DM is puzzling and if real, has no easy explanation in the light of present CDM models. Not only is there a general trend, but several individual features found in the rotation curve seems to correspond to features in gas circular velocity. This can be observed in Fig. 2 for NGC 1560 and Fig. 8 for NGC 2460.
This fact has inspired a theory (commented in section 3.1), identifying the dark matter with an as yet undetected dark gas. The magnetic hypothesis would provide another explanation, as the rotation curve is due in part to magnetic fields, which are generated by gas. A direct relation is not obvious when very extended curves are obtained (Corbelli and Salucci, 1999). Bosma (1998) himself states that this relation may not be correct.