The only centers and boundaries we can logically speak of, is the center of our "observable universe"...which of course anyone can legitimately claim, from wherever he is.
While knowledge and data of BB model only goes back to t+10-43 seconds, cosmologists are able to reasonably speculate re those early times. During those early times, the four forces we know of today were united in what was called the "Superforce" As space expanded and temperatures and pressures dropped, this superforce started to break up or decouple, gravity being the first. This created what we call phase transitions and false vacuums. eg: the phase transition of ice to liquid water.
These false vacuum states may also be responsible for the Inflation epoch.
http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/lec19.html
During this epoch as temperatures and pressures continued to drop, excesses of energy went into creating our very first fundamental particles, quarks, electrons and such. At three minutes the first atomic nucleus was formed [protons and neutrons] The rest is pretty reliable history.
The universe over large scales is homogeneous and isotropic. The same can be applied the the expansion rate...that is, it is only applied over large scales.