first off, “dark matter”(while not fully understood) is measurable (by the rotation of visible matter in galaxies)...
either “dark matter” exists or something totally unknown is causing an effect just like what “dark matter” would cause
so it all seems to boil down to whether Einstein’s equation is correct as written (dark matter exists), or whether it may need an added term (as Einstein himself thought, albeit for a somewhat different reason).
Einstein would have no difficulty with either result.
Right now, most scientists seem to strongly favor the “dark matter” since they can measure a large impact...that “dark matter” so easily explains.........but there’s still room for further discoveries in this arena...
the theories involving extra dimensions mostly arise out of some types of quantum physics (string theories, etc.)....
so far nobody’s detected the extra dimensions, however....
there is a theory that the force of gravity is (believe it or not, relative to the other physical forces) weak.. due to some of its OOmph escaping into another dimension. However, this has not been established, just like the extra dimensions for string theories have not been established.
Certainly extra dimensions COULD exist, but I do not think many scientists take them as an article of faith. What they do is make some equations work better... but this is very indirect evidence that they actually exist
....and (I have not taken a survey but) most scientists would agree that they are THEORETICAL possibilities, not FACTS at this point.
Best,
They’re not measuring anything... they are literally saying this is where matter would have to be for the theory of gravity to be correct as-is, manufacturing it as “dark matter” to make the equations work in defiance of observations. It’s totally circular reasoning. In a sense, “dark matter” is a measurement of how much the theory of gravity is wrong, which on large scales appears to be as much as 97%.