Most of the confusion about the Big Bang revolves around two different incorrect ideas. The first is that the Big Bang theory discusses the origin of the universe. The second is that the Big Bang was an explosion. Here I'll explain what the Big Bang theory really states.
Ever since 1929, astronomers have known the universe is expanding. Edwin Hubble observed that all galaxies except the closest ones to us — which are those in our local group of galaxies — are moving away. In fact, the farther away a galaxy is, the faster it is receding. This expansion isn't motion through space. It's the expansion of space itself.