The evidence that at least some galaxies are dominated by dark matter halos is overwhelming. Attempts to overcome this conclusion by modifying the law of gravity or invoking other processes have failed. Whether all morphological types of galaxy contain DM is not so clear-cut, but the majority opinion supports such a view.
Research in this field seems to be entering a second phase. More critical methods are being employed and many of the simplifying assumptions of the past are being replaced. Theoretical work on maximizing the information that we can obtain from observations that probe DM in galaxies is progressing. Rather than simply establishing whether a particular galaxy has a DM halo, techniques to uncover the extent and shape of halos are becoming more reliable.
While the presence of DM is clear, its nature remains elusive. However, unlike the DM itself, the prospects for uncovering its nature are bright. Microlensing experiments may either detect or exclude baryonic DM in the next few years. Many non-baryonic candidates have already been ruled out, although the remaining parameter space is going to prove difficult to probe. Some observations and theoretical considerations point towards baryonic DM in galaxies, but are not conclusive. It is also quite possible that the DM in galactic halos is a mixture of both baryonic and non-baryonic material. Whatever the solution to this problem, we appear to have the tools and the motivation to find it.
ACKNOWLEDGEMENTS
An enormous number of people have provided valuable help and information during the preparation of this review, ranging from providing me with a constant supply of preprints to careful readings of the draft manuscript. I therefore gratefully acknowledge T. Armandroff, G. Bertin, C. Bird, A. Broeils, G. Byrd, C. Carignan, R. Carlberg, S. Casertano, J. Charlton, G. Da Costa, L. Erickson, S.M. Fall, G. Fahlman, R. Ferlet, K. Freeman, A. Gould, J. Huchra, G. Knapp, J. Kuhn, K. Kuijken, G. Lake, M. Loewenstein, R. Malaney, P. Maloney, M. Mateo, M. Merrifield, D. Merritt, E. Olszewski, M. Persic, B. Phookun, C. Pryor, D. Puche, H. Richer, D. Richstone, V. Rubin, H.-W. Rix, B. Ryden, P. Sackett, P. Salucci, R. Sancisi, T. Smecker-Hane, T. Steiman-Cameron, W. Sullivan, P. Teuben, S. Tremaine, V. Trimble, J. van Gorkom, R. Webster, B. Whitmore, R. Wyse, and D. Zaritsky. I also thank H. Bond for providing just enough pressure to ensure that this review was completed. This work was supported by a postdoctoral fellowship from SERC/NATO.