I'm not a physicist, but my understanding is that exotic matter isn't strictly required. They say that because what's needed is a sort of negative energy/mass

I'm not a physicist, but my understanding is that exotic matter isn't strictly required. They say that because what's needed is a sort of negative energy/mass to cause an anti-gravity effect. But even that isn't accurate. What's really required is a lower energy density than a vacuum. This can be achieved in some cases, for example, in the casimir effect. When you look at it that way, it opens up some possibility, that we could achieve this in some sort if fashion, at least on a small scale, right? Can someone more knowledgeable than me weigh in on this?