Dark matter models are facing increasingly serious difficulties on galaxy scales as the quality of both the simulations and the data constraining the properties of galaxy halos accumulates. Those who suggest that a revised law of gravity in the ultra-weak field limit might be more natural (Milgrom 1983; Sanders 1997; McGaugh & de Blok 1998; Mannheim 2000; and others) are regarded as radicals. However, theories incorporating both a fine-tuned cosmological constant and dark matter particles with ad hoc properties have lost the compelling aesthetic simplicity of the original CDM model.
The problem of formulating a generally covariant, alternative theory of gravity upon which a competing model of structure formation can be built is itself a severe challenge. Without such a theory, we cannot say whether the other problems on our preliminary list in Section 5 will turn out to be harder to resolve than those of Section 3, and it is possible that the list would grow to become still more daunting if such a theory could be developed. But the difficulties facing DM models and the success of the MOND phenomenology suggest that a serious examination of alternative gravity solutions is called for.
Acknowledgements. We thank Paul Steinhardt and Michael Turner for critical readings of a draft version. Comments from Stacy McGaugh, Bob Sanders, David Spergel, Matthias Steinmetz and Scott Tremaine were also helpful. This work was supported by NASA LTSA grant NAG 5-6037 and NASA ATP grant NAG 5-7015. AK is a Cotrell Scholar of the Research Corporation.