It's not a total explanation, but a way to soften the load on ones brain when thinking about this is to remind yourself not to think in linear terms. "Time" most likely doesn't exist outside the physical universe. "Before" the big bang there was no flow of time, so actually you don't need to take into account factors like "what was before", or "what came after". Basically in a situation where no physical law exists there is no before, present or after.
Which shoots me straight back around to how does something come from nothing? how does time exist if time did not exist before? How on earth is "time" as a factor created?