There is not the slightest doubt that the the universe is real. It is three-dimensional.
But one popular alternative notion has been the "holographic principle", which asserts that a mathematical description of the universe only requires two dimensions. What we perceive as three dimensional may just be the image of two dimensional processes on a huge cosmic horizon.
Up until now, this speculation has only been mathematically analyzed in exotic spaces with negative curvature. Math, like any language, can talk about lots of things that are not possible and such spaces are quite different from the space in our own universe.
A new paper suggests that the holographic principle even holds in a flat spacetime.