Negative mass may be like the supposed 100MPG carburetor-- no one can find it because it's been hidden.
https://www.newscientist.com/article/mg23130890-400-metaphysics-special-why-is-there-something-rather-than-nothing/

"German philosopher Martin Heidegger’s angst-ridden non-answer to what he called “the fundamental question” was that a fear of nothing was the defining feature of the human condition. We certainly seem scared that nothing is some kind of universal default. But why should we presume that nothing is more likely than something? After all, if we accept that we exist to ask the question, then we’ve proved something exists. It’s a whole lot harder to prove that nothing can exist."

It seems to me that Heidegger explained both why we keep looking and why we'll never find out.