Things like faster-than-light travel have very good reasons for being believed theoretically impossible (e.g. they allow traveling backwards in time). Things like magnetic monopoles are seemingly allowed to exist theoretically, even if they haven't been found in practice.
My question is whether "anti-mass" (something that has an "opposite effect" of how mass curves spacetime) falls in the category of "theoretically most likely impossible", or in the category of "theoretically admissible" (like magnetic monopoles).
Lastly, is there any evidence of "anti-mass" at all?
The reason I ask is that a bunch of sci-fi concepts seem very impossible, but I've never seen any discussion about "anti-gravity" being theoretically valid or not.
Edit: In particular, I'm not AT ALL referring to anti-matter, which has everything to do with charge and nothing to do with actual mass. This question is not a duplicate, as I'm specifically trying not to refer to "anti-matter". Which is why I called it "anti-mass" (has the opposite effect as mass on spacetime). Please re-open the question.
