It's funny that Dark matter is on there as:

What is the identity of dark matter?[6] Is it a particle? Is it the lightest superpartner (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?

That takes Dark matter's existence for granted, as something we just can't observe. On the fundamental level, we're not certain dark matter exists at all.

Among other things, galaxies spin too fast and should really fly apart from the centripetal force. But they stay together. In order for them to stay together the way they do without any new rules, there has to be more mass than we can detect holding it together. And the amount of mass is often a plurality or a majority of the entire galaxy's mass. But despite its requisite prevasive existance, we find absolutely no other signs of it. We've noticed no patterns in predicting where dark matter should appear or how much of it should be there. It's more or less entirely observation-derived and observation-fit. "How much invisible, non-interactive mass do we need to assume exists to make the physics work out?"

Which doesn't necessarily mean it's not exactly that. But either general relativity is wrong (by which I mean incomplete) or there is the otherwise completely undetectable matter out there that exists in no predictable pattern.

Which one is correct? I have no idea. Both results would be pretty weird, and awesome. But the very existence of Dark matter should be considered the mystery - not its properties. That's putting the cart before the horse.