This is an interesting and open question. It suggests that the universe expanded extremely rapidly during the first stage of the big bang. MAP will test these predictions. The proposed answers to this question are even more speculative than inflation. Other physicists believe that we can only address this question after we unify gravity and quantum mechanics.
String theory is currently our best hope of unifying physics.
Brian Greene has just finished a popular book on string theory that should be out in the next few weeks. It describes the current state of the field. The inflationary model is our best idea on how to extend the big bang theory. In the inflation theory, there is a cosmological constant or vacuum energy term that decays to produce matter. One of the main goals of the MAP mission is to test the theory of inflation.