On October 23, 4004 BC, God decided that there needed to be a Universe, so he made one: this one! Yay, God! Yay, Universe! Yay, Bishop Ussher for figuring this out!
(Orrrrr, one could take the messy view espoused by those nasty Scienticians, who think that some kind of natural processes shaped the entire Universe over the course of 15 billion years. Stupid Scienticians!)