There isn't a container. Imagine a 2-dimensional universe is the surface of a baloon, with a bunch of stars drawn on it. If the baloon is inflated, the stars get further apart, even though they aren't moving into empty space (remember, the air inside and outside the baloon aren't part of the 2d universe). Unlike ink on a baloon, matter has several forces stronger than the baloon holding it close together, so the stars themselves don't expand along with the empty space.