Living in what appears to a three-dimensional world it is rather shocking that there is a consensus amongst most quantum physicists that there are eleven dimensions (as according to the show "Parallel Universes" on the Science Channel). The eleventh dimension is the home of parallel universes and accounts for the weakened force of gravity. Gravity may not be leaking from our universe, but into it from a parallel universe. It just boggles the mind. These strange universes have different natural laws than our own and come in equally bizarre shapes. Some universes are composed solely of energy and have little or no depth to them.
There are such a wealth of possibilities, infinite possibilities when parallel universes come into play. It makes for interesting story lines but is this something the average reader can get their head around? It definitely falls within the realm of hard science fiction.
Forget about aliens, the extraterrestrials will be from the eleventh dimension!