Sideways time
Say there was a Big Bang that started another universe but time ran sideways in that universe instead of forwards and backwards like in our universe. So in other words that universe could last for billions and even trillions of years in its own timeline, but from our timeline it would not even be perceptible because it would be over just as soon as it was begun.
So you know when they say there was no time before the Big Bang... Well, maybe there was time but it was just running sideways.