Suppose that our universe is inside a blackhole. Suppose that the blackhole 
was reponsible for creation of the universe... the gasses compounded and 
caused
the big bang and the black hole. Matter is still being sucked in into our 
universe potentially... and still being expelled into another universe.

Such a blackhole could be a portal to another universe.

There seems to be a blackhole in the center of our universe ?

All intelligent lifeforms will eventually figure it out... and be interested 
in examining the center of the universe.

Therefore it's highly likely that Aliens will be near the center of the 
universe ! ;) :)

Bye,
  Skybuck =D