[naasking]

Nah that already presumes that it's a separate thing. It could be a modification of gravity instead.

[kadoban]

It could at one point have been, but that's basically been ruled out by further observations.

It's hard to say why only _some_ galaxies have their gravity modified without it being caused by some sort of substance.

[naasking]

> It could at one point have been, but that's basically been ruled out by further observations.

It really hasn't. The same standard of evidence that would rule out modified gravity would also rule out dark matter.