No center - all observers appear to be at the center. Distances between galaxies increase, but the sizes of galaxies remain the same. This is because galaxies are gravitationally bound locally and do not share in the global expansion. Hubble Constant Measures the rate of expansion. But, distances are very hard to measure.
Galaxies also have extra motions.
Two galaxies orbiting each other Galaxies orbiting within a cluster of galaxies. Indistinguishable from the general expansion. Most galaxies are receding from us. Measured accurately from spectral lines shifts. Peculiar velocities can affect observed v Mapping the Universe Can map the distribution of galaxies using redshifts.