But its 3D spacetime that is expanding. You could set off in 1 direction, and so long as you went straight you'd end up back where you started.
Just like blowing up the balloon with the 2D universe, for the 2D creatures there is no centre in the 2 dimensions they are aware of.
To be aware of the fact the centre of the baloon lies in a 3rd dimension, you need to be aware of depth.
Moving this concept to a 3D universe. The centre lies in an nth spatial dimension.
Length, width, height, sometihng else.
It acutally can't have centre defined by the 3 dimensions.