. . . think of it as an expanding balloon. If you imagine yourself as a 2D object on the surface of the balloon, there is no centre to the balloon surface. The centre you are looking for is in 'hyper-space' - in 3D. Now ramp up the analogy to 3D/4D and that explains your question . . .
[PS for those in the know around here: Having said that, this doesn't stop there being a concentraiton of the universe mass at one point on the balloon's surface. This will cause local curvature and time-dialation in 3+1 space-time]
[This message has been edited by Tranquility Base, 11-28-2002]