[pdonis]

> I thought grandparent was saying that GR is wrong

No. The dark matter hypothesis is part of our best current model based on GR.

That hypothesis does include the belief that, as far as galaxy rotation curves are concerned, GR corrections to Newtonian gravity are negligible. But that is not the case for other observations that dark matter also accounts for (see below).

> and that Vulcan exists, but it’s made of dark matter.

That is basically what the dark matter hypothesis says, yes: that we don't need to modify the laws of gravity to explain galaxy rotation curves, we just have matter there that we can't see.

If this hypothesis were only introduced to explain galaxy rotation curves, that would be one thing. (It still wouldn't be as easily refutable as the Vulcan hypothesis was, since it's a lot easier for us to rule out the presence of significant additional matter in the solar system than it is in distant galaxies. But that's a side issue.) But it isn't. The dark matter hypothesis also explains many other observations (for example, the amount of structure--gravitational clumping of matter into galaxies and galaxy clusters--as compared with the amount of baryonic matter present based on Big Bang nucleosynthesis (which by itself would be much too small to account for the structure that we see). MOND, OTOH, was introduced solely to explain galaxy rotation curves and still leaves all the other observations that dark matter accounts for unexplained.