  The success of the Lambda-CDM model on large scales does not extend down to
galaxy scales. We list a dozen problems of the dark matter hypothesis, some of
which arise in specific models for the formation of structure in the universe,
while others are generic and require fine tuning in any dark matter theory.
Modifications to the theory, such as adding properties to the DM particles
beyond gravitational interactions, or simply a better understanding of the
physics of galaxy formation, may resolve some problems, but a number of
conspiracies and correlations are unlikely to yield to this approach. The
alternative is that mass discrepancies result from of a non-Newtonian law of
gravity, a hypothesis which avoids many of the more intractable problems of
dark matter. A modified law of gravity is not without formidable difficulties
of its own, but it is no longer obvious that they are any more daunting than
those facing DM.
