Even prior to the big bang all matter was accounted for. In energy. The universe has no end. You know the saying "everything that has a beginning must have an end?" well there is no end here...no beginning either. The universe just is. It always was. Constantly changes form from energy to matter and back and forth. Big bangs happen on a universal time scale all the time.
I'm going to say it one more time - we don't know about the conditions 'before' the big bang. We don't know if all energy was accounted for 'prior' to the big bang (although again, if the big bang created both time and space, then is there such a thing as 'before the big bang'?). At this point in time, any speculation about 'before the big bang' is purely philosophical. We might expect that all matter/energy was accounted for, but it needn't be the case. If there was a beginning to time (maybe there was, maybe there wasn't), then energy could, philosophically speaking, have been created from nothing. This is because conservation laws are only applicable under the existence of time. But I can't stress enough that, even though some physical principles and observations may be referenced, everything about this stage of our universe is entirely and purely speculative and philosophical.
And we certainly don't know that big bangs happen on a universal time scale all the time. It's true that up until about a decade or two ago, physicists thought that a repeating cycle of Big Bang -> Big Crunch -> Big Bang -> Big Crunch ad infinitum was a likely scenario, but that theory was fallen almost completely out of favor in light of new cosmological observations.
Time is relative to light anyways. Everytime we look at the sun we see it in its 8 minutes ago form. Time is different here than it is many light years from here. The edge of the uinerse as we know it is simply a light barrier, the fastest and furthest ligh has reached.
To be more specific, time is relative to reference frame. The statement 'time is relative to light' actually opens up a can of worms. For one, if you could, theoretically speaking, hitch a ride on a beam of light, you would experience the entirety of time in a single, infinitesimal moment. The same is true of any other massless particl, like gluons, and also gravitons (if they exist - we've never observed or measured gravitons or gravitational waves, but we think they should exist). It's true that time is relative and passes at different rates in different places and velocities, but that relatively is not free-form. It follows a very strict set of rules, and we can still speak about things like 'the age of the universe' by determining standard measures of time. The trick here is that that standard measure would be different in different locations/speeds in the same way that time itself is, allowing for a perfect 'translation' if you will.
So the existance of other universes or some larger entity that contains our own universe does not affect the age of what we call 'the universe'.
This kind of stuff makes my head almost explode... I thought the universe was the whole thing.
Edit: TheOneX explained it much more simply and better than i did, so... yeah.
Isn't it supposed to burn out eventually?
The current concensus is that, if our observations are correct, then the universe is likely on a one-way ticket to 'death'. The manner of death, though, is debatable.
It might undergo what's called 'heat-death', where the universe ends up in a thermodynamic equilibrium of maximum entropy. Essentially, there is no free energy left available to.. do.. anything. Perfectly stable particles, if they exist, would be stretched evenly but infinitely thinly.
Another potential death scenario is called the Big Rip (which I think is kind of exciting ). Basically, the idea is that in a dark energy dominated universe (which we think ours currently is, and will continue to be - though it didn't used to be ), the observable universe actually shrinks with time. This is because the expansion of the universe isn't just matter and all moving away from each other, but spacetime itself actually 'stretches'. Once the observable universe becomes smaller than any particular structure (whether that's a galaxy cluster, a galaxy, a star, a planet, an atom, a nucleus, a nucleon, ...), then that structure will be ripped apart because there can be no interaction between its farthest parts. The observable universe at any given point would shrink to nothing, and the universe would essentially cease to exist.