This is not actually true. In fact, all straightforward applications of the dark matter hypothesis, which looked promising at first, have pretty much been ruled out by observational evidence. The plain vanilla model in which there is a single type of thermal dark matter with a mass O(1-100) GeV has been ruled out by observation for almost a decade. And, there are, in fact, modified gravity theories (although not the most well known example of the genre called MOND) that do fit the data better than any of the extant dark matter theories (see, e.g., Moffat's MOG theory), and do so with fewer free parameters in their models, although few modified gravity theories have been tested as rigorously and by as large a group of investigators as the leading dark matter theories have been. This is not to say that an entirely satisfactory and well tested solution on any front exists. But, a lot of the data points which folk wisdom assumes destroyed modified gravity theories (e.g. the Bullet Cluster) do no such thing. Indeed, data points like the Bullet Cluster actually do more harm to dark matter particle theories than to modified gravity theories (many of which can accommodate this observation). Also, just to be clear, there is really no reasonable doubt that phenomena usually attributed to dark matter, that can not be explained with GR (at least as currently interpreted and applied*) and ordinary matter, exist and are pervasive. The phenomena attributed to dark matter can only be explained with some sort of new physics that either involves beyond the Standard Model particles, or forces that have effects different from GR as currently interpreted and applied, or both. These phenomena are by far the most compelling direct observational evidence that the "Core Theory" of GR plus the Standard Model is not complete and that New Physics are necessary to explain what is observed. (In contrast, "dark energy" phenomena can be completely explained to the limits of experimental observation with the cosmological constant of conventional GR.) * There are a couple of promising gravitation based theories that claim that they do not actually modify GR but involve a means of applying GR-like concepts different than the way that the vast majority of researchers in the field apply GR to the analysis of complex systems operationally.