Ludwig Shläfli discovered, at the XIX century, the fourth dimension. In fact, he thought that if the first three dimensions exist, by analogy the fourth dimension should exists.
Then, he demonstrated that a shape in 0 dimension is a side of the first dimension, a shape in first dimension is a side of the second dimension, and a shape in second dimension is a side of the third dimension. So the third dimension’s shape is a side of the fourth dimension’s one.

Hypercube

As an example : If we imagine/represent a square :
The representation of a shape, here a square in 0 dimension is a point.
So the side of a shape in first dimension is always a segment because it’s a set of points.
Consequently, the square’s side in second dimension is a segment.
Hence, the sides of cube are squares in the third dimension.
Finally, in fourth dimension, the cube become an hypercube and by analogy it’s sides are cubes.

Fourth_imension

Thanks to this discovery scientists assume that there are portals in different parts of the universe that act as doors to higher and lower dimensional worlds. This is called the idea of a Multiverse.

Different dimensions