My knowledge on this subject is minimal, so sorry if my question is obvious. But why is the current consensus that there is something called dark matter rather than that our current theory of gravity (general relativity) is wrong?
The answers so far have given evidence for dark matter and general relativity. But (this may seem like a bit of strange thought) are we not doing the same as we did with the aether and the aether drag hypothesis? I.e. there is an observation that does not fit with general relativity what we are doing is assuming that general relativity is right and designing 'add on' theories [i.e. dark matter] to make all other observations fit GR. It seems to me that we are just holding onto general relativity because we are 'use to it'. It is like the saying we are 'looking for life as we know it', who says life can't be something completely different (rhetorical question)? This also goes back to Occam's razor, dark matter cannot be explained without making the whole thing more complicated. Another theory of gravity seems like the most simple approach.