I stand happily corrected after a day of reading. Gravity seems to bind spacetime together on small scales and the expansion is only observable over great distances. This then leads me to question whether expansion is just so feebly small at local scales that we just can't detect it. At any rate, contracting the universe back to the singularity nugget that it came from and calling that "point" the center makes no sense unless you are assuming outright that the universe exists in some higher-dimensional embedding space. The big bang (and expansion that followed) is not an expansion in space but an expansion of space. Since accepted cosmology does not embed our universe in something higher there is no such thing as "center of expansion" nor "center of the universe". IF, however, we are embedded in something higher then yes, there really is a center but it is not in our universe.
If we reverse expansion then density grows, temperature grows, and physics as we know it breaks down...this is exactly why we have no answer as to "why" the big bang happened in the first place. As far as I can tell (now) it appears as though all matter would get closer together, not shrink along with the reversed-expansion.
As things get closer and closer the particles that make everything up become more disturbed. At the quantum scale the force of gravity is unbearably weak so I would imagine that the EM force would repel the charged particles. As space contracts there would be no room for repelled particles to escape to so they would shake violently around the ever decreasing space left. As they shook and jetted about the kinetic energy would result in an increase in thermal energy. As the (tiny) universe heated up particles would begin to dissociate themselves from eachother. In fact, we would reach energy scales that particles physicists would only dream of. We'd revert back to the quark-gluon soup and eventually our physics would break down entirely and we'd ultimately get our singularity, I suppose.
That's all just speculation as I'm by no means a professional cosmologist.