This Feynman diagram describes how subatomic particles behave, image by Joelholdsworth for Wikimedia
Quantum theory is a branch of physics that is important for small objects such as atoms and electrons. On this scale scientists have found some very strange behaviour; objects seem to have a ghost-like ability to appear and disappear, be at two places at once and even teleport from one place to another!
Some scientists think that quantum theory also predicts parallel universes. Although this may sound like science-fiction; quantum theory has been tested many times and has been used to design lots of practical devices such as lasers and the miniature electronic circuits in mobile phones and computers.
Scientists are now researching how to build a new type of computer that uses quantum effects directly; quantum-computers may one day replace your laptop and process information millions of times faster than your home computer.
Others scientists use machines called particle accelerators to a re-create the conditions immediately after the Big Bang. These machines smash beams of particles into each other at very high speed (nearly the speed of light) to understand what role quantum effects played in the early universe.