Okay. It's turtles all the way down; or particles, at least. Where did it begin?
Strictly speaking, in this model, it didn't. The universe had simultaneously existed for a finite amount of time, and also never actually started. This is because of the curious, scale-symmetric properties of the speed of light.
The speed of light - the speed of propagation through spacetime - is both the maximum on speed, and also a throttle on the passage of time. The speed of events at a scale of observation is limited by the speed of light. To make sense of this, imagine a brain the size of our galaxy. Imagine the time it takes a piece of information to travel from one point to another; because of how long it takes information to move, from this superbrain's perspective, events at our scales happen absurdly quickly. It would observe the motion of galaxies as something happening quickly.
The same principle applies when you get smaller in scale, although it is perhaps harder to visualize. The distances are closer; light traverses small distances at the same speed it traverses large distances, so events at a smaller scale happen more quickly, from our perspective. From a sufficiently small-scale perspective, the first few milliseconds of the big bang were, scale-relative, billions of years. Galaxies formed, grew cold, and died. In that time, any beings at that scale would have wondered about when it all started, and where it is all going. They perhaps may have discussed the beings who might have existed in the first few milliseconds of the big bang.
Because the big bang wasn't an event, and it isn't over. Zoom out, in both time and space, and you will eventually reach a point where the universe looks like a dense hot plasma. There was no beginning, even if it has only been going on for a finite amount of time, because the meaningful concept of the relevant scale of time is constantly increasing. There is no end. The big bang, and the universe - because they are the same - are an ongoing process.
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