The real answer is:
No one knows.
Science may never know with certainty what caused the Big Bang.
The Big Bang represents something that in mathematics and science is called a “singularity”. It was infinitesimally small and infinitely dense. It is quite possible; maybe even likely, that the physical laws we understand and use to study things in science don’t work under those conditions.
Many scientists feel that it may not be possible to go back to anything that took place before the Big Bang with any sort of assurances that we can understand what actually happened and what caused it. There are some very smart people trying to figure all of that out. But it may very well be the case that no one will ever present a hypothesis that can be tested.
Without an answer, and with the very real possibility that there will never be an answer, creationists rush in to use the “God of the Gaps” argument again. But in this case there initially seems to be a bit more validity to that argument than is the case with other hypotheses.
One argument goes along with the First Law of Thermodynamics. It goes like this (stated as a “proof” that the Big
Bang was supernaturally created:
> So, you want my proof. There is a lot to go over,
> so let's get started...:
> The first law of thermodynamics states that energy
> is not created or destroyed. It can be changed into
> different forms, but there is no gain or loss of
> energy. Thus, the first law of thermodynamics is
> sometimes called the law of conservation of energy.
> This principle derives from the study of the
> physical properties of energy, and therefore,
> states the condition of energy as it is understood
> in our physical world.
> It is an established scientific law because there
> are no known experimental exceptions to suggest
> that energy can be created or destroyed. It has
> been studied and analyzed by thousands of scientists
> for over two hundred years. Observation shows that
> the universe exists in many forms of energy, such
> as matter, light, and heat, but there are no known
> physical conditions whereby energy can be or is
> created. The obvious question often asked is,
> "Where did this energy come from?"
Wow. That sounds like a pretty powerful argument.
What’s the answer?
One answer, of course, is that since we don’t know what happened before the Big Bang it is quite possible that whatever preceded it had energy.
But there is a better answer. That answer involves the idea of “potential energy”. Potential energy is a form of negative energy.
The best example of potential energy is what we see in a pendulum clock such as a Grandfather clock. In such clocks, you pull a weight up and that elevated weight powers the pendulum. As the pendulum moves back-and-forth the weight descends. If you never pull it back up it will hit the bottom of the column causing the pendulum to stop.
So when you pull the weight up, you are “investing” energy into the clock – creating negative energy. Over time the positive energy needed to keep the pendulum moving works against that stored or positive energy. When the weight hits the bottom of the clock, the net energy is zero.
In fact, potential energy – negative energy – is created whenever masses are moved apart from each other in defiance of gravity. The weight in the clock is moved away from the center of the Earth in order to create this potential to allow the clock to continue to run.
The further that you move masses away from each other, the more energy that is required.
If you factor in all of these potential energies, the total energy of the universe is actually zero!
The famous theoretical physicist Stephen Hawking explains it like this.
"There are something like ten million million million million million million million million million million million million million million (1 with eighty zeroes after it) particles in the region of the universe that we can observe. Where did they all come from? The answer is that, in quantum theory, particles can be created out of energy in the form of particle/antiparticle parts. But that just raises the question of where the energy came from. The answer is that the total energy of the universe is exactly zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity. Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together. Thus in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero."
So things are not quite as mysterious and impossible as we would originally think.
 Stephen Hawking, "A Brief History of Time", p. 129