This is one of the traditional arguments for the existence of God. The fact that something exists today means that something is eternal, for if there ever was a time when there was nothing, then we would still have nothing today, as something cannot come from nothing. The only two options for what can be eternal are the universe itself or something outside of and greater than the universe. And since we now know that the universe is not eternal, it must be the something outside of and greater than the universe, namely God, that is eternal.
Many times skeptics will ask “who made God?” But this is really an improper question, as God exists outside of time, and by definition a timeless entity had no beginning. And why do we say that God exists outside of time? Because we know from Einstein that space and time are related to each other, and that if there is no space (that is, the universe does not yet exist), there is no time. The Big Bang (or God’s stretching out of the heavens) did not occur into pre-existing space, but the expansion itself produced space.