> It would be, by fair, the biggest failure in modern science. So there is reason that alternatives to dark matter are held to a higher standard than dark matter itself, beyond simplicity - which dark matter no longer really is.
I don't think this is anything to do with a higher standard. It's more like Occam's razor. If there's an alternative explanation that will upend our whole understanding of gravity and cosmology, have at it. But such an explanation must come with something testable that shows it's true.
Here's an example of an alternative explanation, "a wizard did it." But that's not science, because there's no way to prove it. As a result, in the absence of a testable hypothesis, you have to go along with the one that does the least violence to all currently existing knowledge, surely?
I.e. what I'm saying is that it isn't on the Dark matter theory to prove itself. We already have observable gravitational issues that need something weird to be true. Dark matter is just the simplest explanation anyone can come up with that fits the facts. It's on any other theory to either throw out less of already-established science by being an even simpler explanation, or to prove it's justified in throwing out more by showing experimental evidence.
Or, to put it another way. I agree that dark matter isn't a simple theory. But there isn't currently a simpler one on the market.