Good Point! The big bang theory is just that...a theory with no concrete evidence to prove it...but...the universe is expanding very slowly. This was proven by taking measurements between galaxies and stars. So...there had to be something to cause this expansion. In space there is no gravity, so objects moving through space won't basically stop until they encounter a gravitational force to cause the to stop. If a cannon ball were launched in space...that ball would almost travel on forever, or, at least until it encountered a gravitational, or magnetic force to pull it in.
So the big bang is a non proven theory, but it is more plausible than the creation theory which also hasn't been proven.