The universe always was
Matter cannot be created or destroyed
How many times do I have to say this? The universe cannot be eternal. It's a scientific impossibility. If it was eternal, there would be no energy left. It would have been used up an eternity ago. Damn! You're stupid.
It is a scientific impossibility for matter to self create itself
Energy is created.....matter is not
In string theory, we have a multiverse of universes. Think of our universe as the surface of a soap bubble, which is expanding. We live on the skin of this bubble. But string theory predicts that there should be other bubbles out there, which can collide with other bubbles or even sprout or bud baby bubbles, as in a bubble bath.
But how can an entire universe come out of nothing? This apparently violates the conservation of matter and energy. But there is a simple answer.
Matter, of course, has positive energy. But gravity has negative energy. (For example, you have to add energy to the earth in order to tear it away from the sun. One separated far from the solar system, the earth then has zero gravitational energy. But this means that the original solar system had negative energy.)
If you do the math, you find out that the sum total of matter in the universe can cancel against the sum total of negative gravitational energy, yielding a universe with zero (or close to zero) net matter/energy. So, in some sense, universes are for free. It does not take net matter and energy to create entire universes. In this way, in the bubble bath, bubbles can collide, create baby bubbles, or simple pop into existence from nothing.
Can a Universe Create Itself Out of Nothing?