I'm definitely no physicist or cosmologist, but my understanding is that we can't detect expansion at the scale of molecules, humans, earth, etc. We only know about it at cosmological scales because we can observe redshifted light over huge distances (millions of light-years). Gravity is postulated to bind matter together strongly enough at smaller distances that expansion does not occur. In other words, it pretty much only happens between galaxy clusters, where gravity is too weak to overpower it.