Well what you're talking about is one of the basic tenets of christianity: that God has existed forever. as you say, matter can neither be created nor destroyed, but it can be changed from one form to another. perhaps God used part of himself to create existence. who knows.

examine the current patterns in cosmology, along with the cosmic microwave background, one really can't conclude with any understanding of science that the universe is more than 20 billion years old. What happened before that, though, is a useless question, since space and time are bound together and before the universe there would have been no time, and hence no before.

matter can be created and destroyed. What's constant is a system's energy. Matter has a stored energy equal its mass multiplied by the square of the speed of light. (hence einsteins equation thingy)