Before listing the problems of DM, we briefly review two properties of the mass distribution within galaxies that exacerbate some of the difficulties. Our conclusions are controversial, but we find strong evidence for both points.
First, putative galaxy halos seem to have cores of finite, as opposed to cusped, central densities. Density estimates from rotation curves of low-luminosity and low surface brightness (SB) galaxies are most revealing, since these galaxies have the largest mass discrepancies and therefore halos for which compression by baryonic infall is least important. Van den Bosch et al. (2000) stress that the inner slope in HI data can be underestimated because of ``beam smearing,'' and Swaters et al. (2000) indeed find that optical data often indicate a steeper rise; but their cases either require, or are consistent with, a finite density core - whatever M/L is ascribed to the luminous component. Other well-resolved rotation curves of low-luminosity galaxies (Rubin et al. 1985; Courteau 1997; Sofue et al. 1999; Blais-Ouellete et al. 1999, b; Palunas & Williams 2000) generally also reveal a gentle rise, indicating that their halos do indeed have low-density cores.
Second, the mass in the inner parts of bright galaxies is dominated by the luminous disk and bulge. ``Maximum disk'' models are strongly suggested by the impressive match between the shapes of inner rotation curves and predictions from the luminous matter alone (Kalnajs 1983; Kent 1986; Broeils & Courteau 1997; Palunas & Williams 2000), and other evidence is reviewed by Bosma (1999) and Freeman (this meeting). Furthermore, recent work on barred galaxies (Debattista & Sellwood 1998, 2000; Weiner et al. 2000) has shown that any DM halo makes a negligible contribution to the inner rotation curve even after the formation of the disk and bulge, and that this conclusion for barred galaxies also holds for their unbarred counterparts.