Open Questions: Alternative theories of gravity

The MOND pages: Collection of links and pages with information on the theory of "Modified Newtonian Dynamics" as an alternative to dark matter for providing an explanation of galaxy rotation curves. In addition to the external links, are frequently-asked questions and critiques of the evidence for dark matter. The pages are maintained by Stacy McGaugh.

Mystery of the missing mini-galaxies: August 2009 New Scientist article. "The Milky Way should be surrounded by thousands of tiny galaxies, but we've only seen about two dozen. That spells trouble for gravity as we know it."
Gravity's dark side: June 2006 article from Physics World, by Matthew Chalmers. "Despite decades of searching, the "dark matter" thought to hold galaxies together is still nowhere to be found. Matthew Chalmers describes how some physicists think it makes more sense to change our theory of gravity instead."

Revising Relativity
Graham P. Colins
Scientific American, November 2002: Scientists try to outdo Einstein.