The past exists in the sense that we can remember the past or examine evidence of past events which occurred. It's my understanding that the Earth is calculated to have existed around 4.5 billion years and the universe itself around 13 billion years (give or take). As to what, if anything, happened before that time, no one seems to know.
But if we're considering all of eternity - an infinite past and an infinite future, then even billions of years would just be a tiny fraction of time.
Eternity is a long time - trillions, quadrillions, quintillions of years. I've heard some believers say that God always existed and that no one actually "created" God. This means that God was ostensibly sitting alone in the dark for quadrillions of years before He decided to create the universe. What was He doing all that time?