'The Hitch Hiker's Guide to the Galaxy has, in what we laughingly call the past, had a great deal to say on the subject of parallel universes. Very little of this is, however, at all comprehensible to anyone below the level of Advanced God, and since it is now well-established that all known gods came into existence a good three millionths of a second after the Universe began rather than, as they usually claimed, the previous week, they already have a great deal of explaining to do as it is, and are therefore not available for comment on matters of deep physics at this time.
One encouraging thing the Guide does have to say on the subject of parallel universes is that you don't stand the remotest chance of understanding it. You can therefore say "What?" and "Eh?" and even go cross-eyes and start to blither if you like without any fear of making a fool of yourself.
The first thing to realize about parallel universes, the Guide says, is that they are not parallel.
It is also important to realize that they are not, strictly speaking, universes either, but it is easiest if you try and realize that a little later, after you've realized that everything you've realized up to that moment is not true.
The reason they are not universes is that any given universe is not actually a thing as such, but is just a way of looking at what is generally known as the WSOGMM, or Whole Sort of General Mish Mash. The Whole Sort of General Mish Mash doesn't actually exist either, but it is just the sum total of all the different ways there would be of looking at it if it did.
The reason they are not parallel is the same reason that the sea is not parallel. It doesn't mean anything. You can slice the Whole Sort of General Mish Mash any way you like and you will generally come up with something that someone will call home.
Please feel free to blither now.'