we know of the law of cause and effect, so if the Big Bang theory is correct what was the cause?

I think the honest answer is that we don't know. We can talk about quantum fluctuations, but there is no way to currently understand what occurred prior to the Big Bang, or even if there is such a thing as "prior" - since time probably originated along with space at the event. If time and space both originated there, then cause and effect may not apply, since it is based on time and space being real. The theory does not address this.

Everything grew out of nothing, or what we would consider to be nothing, which was really the vast expanse of the universe.  As energy is never created or destroyed, all the energy of the universe already existed, but in a different form.  The "Big Bang" was caused by the energy of the universe expanding and changing form.  

The laws of physics were products of the big bang.  Therefore, there's no basis to assume they had to apply to the big bang.  There needn't, for example, have been a cause.  It's also known that, at the quantum level, not all effects appear to have causes.

Some say a big force field of energy.

God?

Big Bang theory was coined by Fred Hoyle who was very skeptical about the Idea.  Its only a Theory which leaves a lot to be desired. Never the Less it is a Theory that indicates that the Universe and  its Time Period had a beginning. Not much is said about a Time less system or structure that has no time,or  beginning, or boundary.

As the How,why and what Caused the Structure  of The Universe as we observe it to be, is only information that belongs to Our Creator.

The problem with cause and effect is:

1.  It requires a background of time in which to make any sense:  The cause is always supposed to come before the effect.

2.  All physical calculations and predictions become meaningless at the Singularity. For example, as you approach the Ultimate Singularity at the exact "event" of the "Beginning" of the Big Bang (or, if you like, the singularity of any black hole) Time and Space and I think even Energy and Mass become totally fluctuant and interchangeable, and all physical calculations just break down with infinities and zeros popping up all over the place.

3. Like everything in our universe, Time starts at the Big Bang.  So thinking about causes that gave rise to the Big Bang (ie events that would have happened at a time before the Big Bang) is like the proverbial example of thinking about what is 'North of the North Pole".

I know this is a convoluted answer; but I hope this makes some sense.

Three things:

1) The laws of physics are interconnected with space, matter and energy as it currently exists.  Prior to the big bang, these laws of physics may very well not have existed, meaning that cause and effect doesn't necessarily apply.  It is believed that the ordinary laws of physics break down within a singularity because it has no mass for these laws to act upon.

2) Who is to say that it didn't have a cause?  We may not know what it was, but we do know that the big bang DID happen, based on photos taken of distant portions of the universe, the red shift of other galaxies and background radiation being so uniform.  We don't fully understand the exact nature of the event, but we do know it happened.

3) There are a ton of hypotheses about possible causes.  Some have large objects colliding, others suggest that black holes form daughter universe, some involve intelligent entities manufacturing or influencing the big bang, and so on.