Our universe is unimaginably big. Hundreds of billions – if not trillions – of galaxies spin through space, each containing billions or trillions of stars. Some researchers studying models of the universe speculate that the universe’s diameter could be 7 billion light years across. Others think it could be infinite. But is it all that’s out there?

Science fiction loves the idea of parallel universes and the thought that we might be living just one of an infinite number of possible lives. Multiverses aren’t reserved for science fiction, though. Real scientific theory explores – and in some cases supports – the case for universes outside, parallel to or distant from but mirroring our own. Multiverses and parallel worlds are often argued in the context of other major scientific concepts like the Big Bang, string theory and quantum mechanics.

Around 13.8 billion years ago, everything we know of was an infinitesimal singularity. Then, according to the Big Bang theory, the universe burst into action, inflating faster than the speed of light in all directions for a tiny fraction of a second. Before 10 -32 seconds had passed, the universe had exploded outwards to 10 times its original size in a process called cosmic inflation. And that’s all before the actual expansion of matter that we usually think of as the Big Bang itself, which was a consequence of all this inflation. As the inflation slowed, a flood of matter and radiation appeared – creating the classic Big Bang fireball – and began to form the atoms, molecules, stars and galaxies that populate the vastness of space that surrounds us.

That mysterious process of inflation and the Big Bang have convinced some researchers that multiple universes are possible, or even very likely. According to theoretical physicist Alexander Vilenkin of Tufts University in Massachusetts, inflation didn’t end everywhere at the same time. While it ended for everything that we can detect from Earth 13.8 billion years ago, cosmic inflation continues in other places. This is called the theory of eternal inflation. And as inflation ends in a particular place, a new bubble universe forms, Vilenkin said in 2011.