However I do feel the question of what triggered/caused the intial burst of inflation (what some people refer to as the ''bang'') will be much more difficult to answer.
One scientific theory explains how the vacuum energy spontaneously splits into matter/anti-matter all around us all of the time, but gravity then pulls them together and they convert back into energy. Hawkins theorised that because anti-matter is heavier than matter any occurrence close the the event horizon of a black hole would result in the matter escaping and the anti-matter being pulled into the black hole; this anti-matter would make the black hole less dense and over time would result in the black hole exploding and releasing its matter into the universe. If I understand correctly this has now been observed in the form of black hole radiation.
I don't think it would be too far fetched to think that some derivative of this (where matter clumps together) could build up over a very long time forming an incredibly massive black hole which eventually would explode and create a universe. Once the matter in our universe has spread out and died its cold death there will be nothing but a massive empty space for this to happen again - it could already be happening elsewhere.
I don't think the "cause" of the expansion is beyond us, I think we will discover it within my lifetime (Insha anna)

What the cause the vacuum energy is though is a much tougher question I suspect.
I don't think we have any right to ask where God came from
Why not? Do you fear that creator god will be offended? You are presuming that creator god has feelings, and that it is intelligent. Creator god might be a one-trick-pony capable only of creating universes and nothing else (no compassion, conversation, nothing). Creator god may even have been a one-hit wonder and ceased to exist at the point the current formation of the universe occurred.
In short I suspect you feel you have no right to question because you see creator god as a figure of authority who might be easily angered or upset. This is due to a claim of knowledge about creator god which you cannot substantiate. How for example do you know that creator god isn't already upset that we worked out how to split the atom, or how to split the frequencies of light through a prism? Maybe creator god wants all of its creation to be a secret and not just its own origin. If such a thing exists you have no way of knowing what upsets it or even if it is capable of being / willing to be upset.
So with that in mind, what's wrong with "Where did creator god come from?" Maybe this is the only thing creator god cannot work out, so it created a massive logic processor in the form of the universe in order to become aware of itself, then aware of its creator, and then ultimate the source of the creators creation? If we refuse to find the answer maybe creator god will scrap the current universe as a failed prototype and start again with a new one.
Without answers you cannot know which questions are offensive.