Cosmological Principle

Application of the theory of general relativity to the large-scale structure of the Universe leads to various cosmological theories. Of these, the most important are a class of solutions to Einstein's equations called Friedmann cosmologies that lead to the hot big bang. The starting point for these solutions is the cosmological principle:

The Cosmological Principle
Viewed on sufficiently large distance scales, there are no preferred directions or preferred places in the Universe.

Stated simply, this principle means that averaged over large enough distances, one part of the Universe looks approximately like any other part. The cosmological principle clearly is not true on small distance scales: your classroom probably looks rather different from your dormitory room, and we certainly see distinguishing structure like galactic superclusters and the Great Wall up to at least 100-200 Mpc scales. However, well beyond those distance scales there is strong observational evidence for the cosmological principle (see the discussion of relative scales in the right frame).

References:
hot big bang
cosmological principle