With the M-Theory model, travel amongst two "universes" isn't possible. If there were to be extra-terrestrials that are capable of traveling to our planet, they would have to be extremely technologically advanced. Most people tend to forget how vast the universe is. It takes light approximately 5 years to travel to alpha centauri (the closest star to the sun). Relativity holds that nothing can travel faster than the speed of light. When accounting for the momentum of an object as massive as a space ship, it would take an astronomical amount of energy to take a space ship to travel to another place inhabited by intelligent life in a plausible time frame traveling with a high velocity.