[smolder]

I don't think your bacteria on a slide analogy makes much sense. A thin 3D space is still not 2D. A bacterium also doesn't have any comprehension or concept of anything, so of course it doesn't have a concept of 3D space.

Undetectable beings living in another part of reality we're unaware of are, of course, unfalsifiable.

[tomohelix]

From what I remembered, string theory proposed that the extra dimensions beyond the spatial and chronological ones we experience are also compactified and technically hidden from our perception. In another word, they exist just not on a scale we can appreciate. If you accept that then the 2D plane with the 3rd dimension minimized to the point of irrelevance is not so different.

And I don't really care about what the bacteria "think". The point being there exists an extradimensional relationship between me and the bacteria right here in front of me. So at least to me, it isn't so outlandish that there can be something above this 3D plane and I am simply too primitive to understand it.

Of course, there is no sufficient proof right now so I do not accept it as a real answer. It is simply analogical reasoning. Also, I would not say it is unfalsifiable. Like mentioned earlier, the extra dimensions are simply tucked away and not easy to detect. They still exist and with sufficient technology, they can be probed.