| MadSci Network: Astronomy |
Hi Shane,
There are actually a number of reasons why astronomers believe firmly in the existence of dark matter, and not that something is wrong with our understanding of gravity.
It's true that Newton's law of gravity was originally an empirical formulation. It has one enormous strength, however--it works! At least so long as the gravitational field isn't so strong as to put you into the relativistic regime. Ever since it was first formulated, Newton's law has been used to calculate very accurate orbits of the planets. Einstein's general theory of relativity was originally formulated on a much more theoretical basis than was Newton's law. It, too, however, has passed every observational and experimental test to which it has been put.
In addition to passing observational tests, we have good theoretical reasons to believe that gravity should behave in the way described by relativity, based either on the warped spacetime concept of relativity or upon the concepts being developed by attempts to describe gravity using quantum mechanics. We have no reason, therefore, to believe that gravity should behave very differently from the manner described by physical theory.
It is possible to explain at least some of the "dark matter" observations by assuming that gravity doesn't behave quite in the Newtonian or relativistic fashion at large distances, and some attempts have been made to do this. The changes that have to be made to the gravitational laws, however, are pretty arbitrary, and have no good theoretical basis. It seems much simpler, therefore, to believe that the measurements made by astronomers are affected by the presence of unseen matter. After all, it would be incredibly biased to assume that everything in the universe is in a form easily observable by us, and there are any number of candidates for objects or forms of matter that aren't.