Nothing

Religious apologists claim that our universe came from nothing and, thus, requires a God to explain it.  However, physicist Lawrence Krauss, has posited that "nothing" is really "something" in his book, A Universe from Nothing:  Why There is Something Rather Than Nothing.

The following is from an interview of Krauss by Ira Flatlow on NPR's Science Friday:

" - - - I guess most importantly that the question why is there something rather than nothing is really a scientific question, not a religious or philosophical question, because both nothing and something are scientific concepts, and our discoveries over the past 30 years have completely changed what we mean by nothing.

"In particular, nothing is unstable. Nothing can create something all the time due to the laws of quantum mechanics, and it's - it's fascinatingly interesting. And what I wanted to do was use the hook of this question, which I think as I say has provoked religious people, as well as scientists, to encourage people to try and understand the amazing universe that we actually live in."

"Empty space is a boiling, bubbling brew of virtual particles that pop in and out of existence in a time scale so short that you can't even measure them. Now, that sounds of course like counting angels on the head of a pin; if you can't measure them, then it doesn't sound like it's science, but in fact you can't measure them directly.

"But we can measure their effects indirectly. These particles that are popping in and out of existence actually affect the properties of atoms and nuclei and actually are responsible for most of the mass inside your body. And in fact, really one of the things that motivated this book was the most profound discovery in recent times, and you even alluded to it in the last segment, the discovery that most of the energy of the universe actually resides in empty space. (Dark Energy)

"You take space, get rid of all the particles, all the radiation, and it actually carries energy, and that notion that in fact empty space - once you allow gravity into the game, what seems impossible is possible. It sounds like it would violate the conservation of energy for you to start with nothing and end up with lots of stuff, but the great thing about gravity is it's a little trickier.

"Gravity allows positive energy and negative energy, and out of nothing you can create positive energy particles, and as long as a gravitational attraction produces enough negative energy, the sum of their energy can be zero. And in fact when we look out at the universe and try and measure its total energy, we come up with zero."

" - - -  the amazing thing is, once you apply in fact quantum mechanics to gravity, as you were beginning to allude again in the last segment, then it's possible, in fact it's implied, that space itself can be created where there was nothing before, that literally whole universes can pop out of nothing by the laws of quantum mechanics."

" - - - nothing is unstable. It will always produce something."

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Yes, it is speculation.  However, it is plausible and certainly more explanatory than "God did it."