What does a flat universe mean, is it not in 3D? A picture says a thousand words. When you understand these pictures, you will know what is referred to as the flat universe, flat meaning two dimensional. The universe is topologically flat. Topology is the “mathematical study of the properties that are preserved through deformations, twisting, and stretching of objects. Tearing, however, is not allowed,” lifted from here: Topology -- from Wolfram MathWorld I am not referring to any physical explanation because the material world is always morphing. Every time one takes a breath, one’s physical state changes but one’s personal identity remains exactly the same. In this sense, your physical state changes from moment to moment depending on a variety of physical factors. The universe is a torus. The galaxies are two dimensional objects that move on the surface of the doughnut shaped universe. The picture below shows the bombshell. A rectangle and a torus can be transformed into each other.