I can think of two reasons: 1. Expansion is very small, and therefore not noticeable. That is, space expands by 0.00005% in a year (which does not rule out the universe expanding by say, a million light years in a year), so we don't feel it. 2. Everything expands, so we don't notice it. But then... how did we discover it? Perhaps I'm partially right, but can someone give a thourough explanation?