The "Big Bang Theory" is the most sufficient explanation available, unless you have an improvement to offer.
Many races believe that the Universe was created by some sort of god or in the Big Bang. The Jatravartids, however, believe that the Universe was sneezed out of the nose of a being called the Great Green Arkleseizure. They live in perpetual fear of the time they call “The Coming of the Great White Handkerchief” (their version of the End of the Universe). The theory of the Great Green Arkleseizure is not widely accepted outside Viltvodle VI.