The dwarf irregular galaxies are particularly useful for DM studies since some of them are extremely gas-rich. In such galaxies, the HI rotation curve can be traced to a large number of optical scale-lengths. Indeed, recent evidence suggests that in at least one dwarf irregular galaxy the observed rotation curve is approaching the edge of the dark halo. There is no sharp distinction between bright dwarf irregulars and faint late-type spirals, so the demarcation between the contents of this Section and the next is necessarily somewhat arbitrary.
The techniques used to infer the presence and amount of DM in dwarf irregular and spiral galaxies are also similar. Typically, an HI rotation curve and the virial theorem provide an estimate of the total gravitating mass within the last measured point of the rotation curve. The fraction of this mass provided by the visible stars is found from optical photometry. In dwarf irregulars, however, the gas mass is often substantial and must be included in the inventory. The gas mass is also obtained from the HI observations, along with measurements of H2 content when available.
One of the most striking results to emerge from recent studies of dwarf irregulars is that at least some of them appear to have much higher DM fractions than normal bright spirals. Related to this is the observation that the dark halo of dwarf irregulars often dominates the dynamics even within the optical radius of the galaxy, whereas in bright spirals the visible disk produces at least half the rotation velocity at small radii. Both trends are similar to those found for the dwarf spheroidal galaxies described in the previous section. Before discussing these general trends, however, it is instructive to describe some of the observations of individual galaxies.