Have you heard the Many Worlds Theory? The one where every decision you make (every decision you've ever made) splits off the universe. So, if you decide to have a bagel for breakfast with your coffee instead of your usual plain coffee, there's another universe where you just had the coffee.
It's a hard concept to get one's head around. This sort of explains the idea.
I've never been fond of this idea. But it's the jump off point for today's question:
What if, instead of separate universes "breaking off" at every decision point, every possible decision you could have made is being played out in this universe, just amongst different people? That is, you could find out what would have happened if you made a different choice if you could find the person who made that other choice.