Galaxy rotation under the influence of dark matter?

Primary evidence on the existence of "dark matter". On the left is the Keplerian prediction of galactic motion; on the right is what is observed. But what if gravity was not constant throughout the cross section of the galaxy and perhaps was a function (possibly an inverse function) of the local mass density? Kepler's laws with a variable G can then be incorporated into the MOND (Modified Newtonian Dynamics) theories. There is an absence of any evidence of dark matter and the current lack of understanding of gravity indicates that further research needs to first solve the puzzle of gravity prior to trying to prove the existence of dark matter. In other words, how can we prove the existence of dark matter, when we do not yet understand the interactions of matter in general?