Because it is thought that the Universe is like the surface of a balloon, exactly the same that soap bubbles. The Universe is only that surface, nothing inside and nothing outside that surface counts. And it is in an expansion, just like when a bubble is formed and expands. The universe has clusters and superclusters that are located along thin bubble walls. The interior of the bubbles is voided. It can take millions of years to cross those voids. The galaxies are distributed in a way that they look like a 2D web or 3D bubbles.
