Dark MATTER. We are reasonably sure it is matter because is has mass. It isnt totally unobserved. It is a thing causing an observable effect on other matter. In that sence it is no less real than the black holes that we only observe via thier effects on nearby matter.
A correction: based on the model that gravity is the only significant force in operation at the galactic and intergalactic level and that the stars within a galaxy aren't moving in accordance with the model, it is being assumed that there is some amount of unseen and unobservable something that is supposed to have mass and is causing the anomalous movement by the stars within the galaxy.
That is the correct presentation. No test yet has occurred to demonstrate that any such "dark" matter exists. That is, all experimental tests have come up nil.
If gravity (and it may be a very big if) is not the only significant force operating at galactic and intergalactic distances then "dark" matter is no longer required.
The assumption that gravity is the only significant force at these distances is just that, an assumption. It may be correct, but on the basis of the observable anomalous behaviours of stars, there is a good likelihood that the assumption is wrong.
There is nothing wrong with the assumption being wrong, it just means that we should be investigating other possible reasons for what is going on.
