It's a 4D spacetime, not a 3D space. Start with a tiny sphere. Expand it. The sphere gets bigger, but no point on the 2D-like surface is the centre. It's like

It's a 4D spacetime, not a 3D space. Start with a tiny sphere. Expand it. The sphere gets bigger, but no point on the 2D-like surface is the centre. It's like that, but the surface is a 4D spacetime. You can't use your imagination to visualise this. But you can use math to describe it. Whether there's some kind of ultradimensional hypercentre is a different question. Even if there was, it wouldn't be accessible from this universe. And don't forget we don't have a clue what spacetime is. Relativity has some nice descriptions of what it does, but there's no fundamental explanation of how the universe generates the phenomena we call position, time, and distance.