  Newtonian mechanics indicates that galaxies and galaxy clusters are much more
massive than we would have guessed from their luminosities, with the
discrepancy being generally attributed to dark matter halos. An alternative
hypothesis is that accelerations in very weak gravitational fields are larger
than predicted by Newton's laws, and there is no need for dark matter. Even
though we do not currently have a satisfactory theory associated with this
rival hypothesis, we can ask whether any observational tests could rule it out
or prefer it over the dark matter hypothesis. Current evidence suggests that
neither hypothesis enjoys a decisive advantage over the other. If dark matter
turns out to be the correct interpretation however, then theories of galaxy
formation face some quite severe fine-tuning problems.
