It doesn't come from nothing but from a parallel universe...
At the center of this parallel universe was a huge and massive black hole, attracting all the substance, and at last, this universe crunched on it-self (my own vision, inspired on the "big crunch theory" of the expansion of our universe). Then, all this substance was sending to our universe in a big bang.
Seriously, some theories say black holes at centers of galaxies are way to another parallel universes. And they are also different theories about the expansion of the universe : infinite expansion, or expansion until a point (which one?), or "big crunch theory".
In all cases, i don't trust in a divine creation, i think all is cyclic.