Balance

What I've often wondered is how is it determined which universe we are in at the present?

First of all, think of time as being like a conveyor belt, infinitely reciprocating, with a bar somewhere on it. Only because it always feels like the present to a human being, the bar does not move its place on the conveyor. But it may seem or look like it is moving because it is relative to the moving conveyor belt.

I've come to the conclusion that life, the cosmos, and all, must be balanced. It always has perfect balance. If the world were to be unbalanced, it would literally go off kilter and we'd seize to exist. But by balanced, I meant more that there needs to be a balance of good and bad in the world. So naturally nothing can be all good or all bad, but somewhere in between. This doesn't mean that something is <50% good and >50% bad, or vice versa, because that would mean that it is mostly either good or bad. Rather, there is no way of separating the good qualities of something and the bad. (For example, one man's trash is another man's treasure. So the trash/treasure can be seen as "good" or "bad", yet it is still the same thing.) The way it is is just the way it has to be in order for the cosmos to remain in balance.
This belief gives me the confidence to go forward, to not worry so much about what may go wrong. I know that whenever something bad happens, it's so that the next time something good happens, I can appreciate it that much better. Like when you go up on a roller coaster just so that you can be that high in order to experience the drop. Or like this quote by Demetri Martin:
"The problem with seeing everything through rose-coloured glasses is that you can't see roses. The glasses cancel them out. You just see stems…"
Or like the saying "One cannot experience true happiness, without having first experienced true sadness."
And if ever something gets thrown off balance in our lives, our conscious mind travels to a different parallel universe. If we experience too much sadness, and commit suicide, our conscious mind doesn't die. It gets transported without our noticing to a different universe, one that is more in balance.
So really, whichever universe we are in right now, it is the one that is most balanced, the centre of the cosmos.
That's not to say we should live our lives recklessly, expecting nothing bad to happen to us. It means that I expect both good and bad things to happen, and I don't have to get depressed about having no control over the bad things. If I make what seems like a "bad" decision at some point (which will inevitably happen) it will make all successive "good" decisions afterward seem even better because "good" is only "good" relate to "bad".
I know that the concept Ma'at (an ancient Egyptian concept meaning the way and order of the universe) has everything under control. I am free to act however I want to, but I do not have the power to mess up the balance of the cosmos. I am simply a part of it, as is everyone else.

Parallel Universes

In my opinion, there are an infinite number of universes that exist relative to this one (the one we are currently in). What does this mean? It means that there are copies of ourselves, and the world, that live a similar life next to ours. There is no way to travel to these universes, for they exist in a different space-time, a different dimension than this.
Have you ever thought about something as simple and classic as flipping a coin? I mean, everyone learned in school what probability means and that there is a fifty-fifty chance of a coin landing on heads or tails, but have you ever wondered why at one particular coin flip, it just happened to land on one side instead of the other? What I think is really happening is that every time someone flips a coin, the same coin (or a copy of it, depending on how you look at it) is being flipped in a parallel universe from this one. In one universe, it lands on heads. In the other, tails. Since we are in one universe at a time, we see only one outcome.
So if the cosmos is composed of infinite parallel universes, then it would be shaped like a tree diagram. The concept of infinity would be difficult to picture, so let's take a look at what just one part of the cosmos might look like. It might have two universes, side by side, each exactly the same except for one tiny detail that is changed. Each splits off, becoming more and more different from the previous, as time goes on in the universes. In the real world this would look like maybe a lost letter that causes someone to never see each other again. Up until the point in time the letter is either lost or found, the two universes are the same. Then the outcome of the letter shapes the future, causing people to go down a different path.