It's well known that all of our evidence for dark matter (and dark energy, too, but that's not the subject here) at the present time is indirect: It comes from observing the gravitational influence of the hypothetical stuff, not from detecting it "directly" (i.e., using some interaction other than gravitational).
So it's natural to ask whether we can do away with dark matter by positing some modification of the behavior of gravity; I've certainly wondered that myself. And it may very well turn out that the behavior of gravity on large scales does not precisely match the prediction of ordinary general relativity. Nevertheless, I think that, by now, we've accumulated enough data to conclude that the universe cannot be explained solely by modifying gravity; there is ample evidence of gravitational forces pointing in directions where there isn't any (ordinary) "stuff" to create them, leading us to accept the existence of ...
