This means that there’s probably a Universe where you were never born, but everything else is the same. Or perhaps your mother had extra brothers and sisters for you, in which case your family is much larger. Now, this can be compared to sitting in a room, listening to radio. When you listen to radio, you hear many frequencies. They exist simultaneously all around you in the room. However, your radio is only tuned to one frequency. In the same way, in your living room, there is the wave function of dinosaurs. There is the wave function of aliens from outer space. There is the wave function of the Roman Empire, because it never fell, 1500 years ago.
All of this coexists inside your living room. However, just like you can only tune into one radio channel, you can only tune into one reality channel, and that is the channel that you exist in. So, in some sense it is true that we coexist with all possible universes. The catch is, we cannot communicate with them, we cannot enter these universes.
However, I personally believe that at some point in the future, that may be our only salvation. The latest cosmological data indicates that the Universe is accelerating, not slowing down, which means the Universe will eventually hit a Big Freeze, trillions of years from now, when temperatures are so low that it will be impossible to have any intelligent being survive.
When the Universe dies, there’s one and only one way to survive in a freezing Universe, and that is to leave the Universe. In evolution, there is a law of biology that says if the environment becomes hostile, either you adapt, you leave, or you die.
When the Universe freezes and temperatures reach near absolute zero, you cannot adapt. The laws of thermodynamics are quite rigid on this question. Either you will die, or you will leave. This means, of course, that we have to create machines that will allow us to enter eleven-dimensional hyperspace. This is still quite speculative, but String theory, in some sense, may be our only salvation. For advanced civilizations in outer space, either we leave or we die. 
- http://www.kurzweilai.net/parallel-universes-the-matrix-and-superintelligence [↩]