To quickly explain it to you (in my humble, layman's understanding!), the multi-verse theory posits that our universe is just one of many, many, many trillion-gajillions; ours is just one in a sea of universes so large that it makes the mind-boggling number of stars in our own universe seem like the number five. What are these other universes - what do they contain? Some scientists theorise that they are different realities and different possibilities of "what could be/what could've been". Some of these universes are supposed to be very akin to ours; others, absolutely different, with an entirely different set of physical laws holding them together (I'm supposing in universes where their basic laws of physics are very different from ours, there may be square planets, or incorporeal life, or no matter exists, or only anti-matter exists, or something so bizarre and odd, not even the most imaginative of us can possibly visualise the type of reality that exists there). Different universes, with different realities - utterly mind-shattering!
Within the multi-verse theory lies the assumption of parallel universes, where different realities - similar to our own, yet, different - are happening all at once. That is to say: the possibilities which we didn't choose in this universe are happening in another. For instance, in this universe/reality, I have chosen to sit here and type this blog; but in another universe/reality, I am reading a book, or driving, or even typing this blog, but chose a slightly different title to head this article. In another universe/reality, I'm a Nobel Laureate recipient. In another universe/reality, Hitler succeeded at his final solution for the Jews. In another universe/reality, Kim Jong Il rules the world. In another universe/reality, everything is upside down, and what we think is negative in this universe/reality, is positive in that universe/reality. The multi-verse theory puts forward a theory which assumes that there are so many possible options of realities. For every action we choose to not do, or for every thought we don't express, there is a reality where it is happening. All combinations of possibilities are probable and expressed in these alternate universes/realities.
Sometimes I sit and think about this multi-verse theory and wonder about the other "me's" who probably exist. There may be billions of them - some very similar to me, others quite dissimilar (not only in terms of life-paths, but also in terms of personalities and character traits). Are they me? Are they just different expressions of myself, and still, integrally connected to me somehow? Or are they not me, and if they fell into my universe/reality would there be some sort of conundrum where our very existence is so diametrically opposed that we have to try to kill the other (only one can live! - cue the Hollywood soundtrack)? Very ominous, indeed, to think about these other "me's" who aren't quite Me. I'm sure that in another universe/reality there's one of me who's uber-successful at writing and has fulfilled all his dreams. Yet, in another universe/reality, there's also a very probable me who died at the age of nineteen, or is a bum on the street with no one to love me. There must even be a universe/reality where I was never born.
In an odd way, the conception of other me's gives Me a sense of warmth and gratefulness. I'm happy to imagine that in another universe/reality I'm the successful person I'm desperately striving to be in this life; but, more importantly, at the same time, the idea of another me makes Me grateful to know that I am living this life with all the wonderful people who constitute my life and who contribute to my life. I wouldn't trade my parents for any other; or my relatives for any other (ok - maybe some of them I'll trade, but they're mostly tolerable); or my friends for any other; or even my life experiences for any other. The weird thing about the multi-verse theory is that it makes me so happy to be Me, and not a version of myself who's unhappy to be me, or even a version of myself who's happy but not blessed with the experiences and people I've had/have.
Of course, at the end of the day, the multi-verse theory (at least at present and as far as I know) is just that: a theory. We are only sure of this life we're living and so, while it's nice to fantasise about super-happy/uber-successful versions of ourselves, that's not the universe/reality we have to live in, so let's all "It's a wonderful life" things and get back to reality and realise: we've got it pretty good.