That’s like asking where the population comes from which streams into London every rush hour and vanishes in the evening. Somewhere else. Yes, could be other worlds like ours - that’s an easy idea to consider now that we know that most of the stars in the sky have exoplanets orbiting them.

Also many of these religions also have the idea that there may be many other realms of existence. That we see the world in a particular way and that there may be others who see our universe in ways that are so different from the way we do, that we can’t even see them, and maybe they can’t see us either. For instance there may be realms of pure thought where you don’t even need a body - the idea of a body made of light, and the idea of not needing a body at all, just pure thought.

If you think that has to be impossible for scientific reasons, well, science itself is part of the world we find ourselves in. Suppose you had a dream, so vivid that you can’t know if you are awake or sleeping? Some people do dream as vividly as that. The famous Nobel prize winning scientist Richard Feynmann was one of those who describes having very vivid dreams, as vivid as real life, and as scientifically detailed too. Well you could be a scientist in that dream. If you are a scientist in real life, it may even be quite likely that you dream that you are a scientist. When you wake up - what has happened to all your dream scientific instruments, calculations and theories? Not saying that this is a dream, but reflecting on that might help one to understand the views of those who think science doesn’t answer all questions of this nature. At least, not yet in its current form. Will it ever?

Also is it possible for an animal or bird or insect to be born as a human? Many of those who believe in reincarnation think that this is possible. If so, well humans are just a tiny fraction of the beings there are even on our world.

But what about the past before there was any life on Earth? If you believe in reincarnation and think it has been going on pretty much endlessly - then it’s not much different whether you go back to the beginning of our Earth or the beginning of our universe - means there must have been something previous to that. So what form of life did any of us take before there was life on Earth, or indeed even anywhere in our universe? Or did it have a beginning somehow? Or is that question itself somehow missing the point?

Their mythologies do actually address that, they talk about times when the entire world system is destroyed, and then it forms again. It’s a bit like the transition from one life to another for an individual. So the cosmos forms again, the beings within it also, and there is some kind of continuity from the previous cosmos, a causal continuity.

And anyway - how does time work? Do you have to be reborn the instant you die? Or weeks later after spending some time in an intermediate state? Can you go into some kind of limbo state, reborn millions of years later? Does time even run the same way for you after you die and before you are reborn, if there are intermediate states?

So, it leaves many questions open for sure. But that’s true of all the ideas about what happens when we die.

I’m Buddhist myself, and in Buddhist teachings then the most important thing is to have an open mind about what happens when one dies, as about everything else. Indeed Buddha warned that spending too much time on some of those questions which we can’t answer can embroil you in “a thicket of views, a wilderness of views, a contortion of views, a writhing of views, a fetter of views”

"This is how he attends inappropriately: 'Was I in the past? Was I not in the past? What was I in the past? How was I in the past? Having been what, what was I in the past? Shall I be in the future? Shall I not be in the future? What shall I be in the future? How shall I be in the future? Having been what, what shall I be in the future?' Or else he is inwardly perplexed about the immediate present: 'Am I? Am I not? What am I? How am I? Where has this being come from? Where is it bound?'

"As he attends inappropriately in this way, one of six kinds of view arises in him: The view I have a self arises in him as true & established, or the view I have no self... or the view It is precisely by means of self that I perceive self... or the view It is precisely by means of self that I perceive not-self... or the view It is precisely by means of not-self that I perceive self arises in him as true & established, or else he has a view like this: This very self of mine — the knower that is sensitive here & there to the ripening of good & bad actions — is the self of mine that is constant, everlasting, eternal, not subject to change, and will stay just as it is for eternity. This is called a thicket of views, a wilderness of views, a contortion of views, a writhing of views, a fetter of views. Bound by a fetter of views, the uninstructed run-of-the-mill person is not freed from birth, aging, & death, from sorrow, lamentation, pain, distress, & despair. He is not freed, I tell you, from suffering & stress.

"The well-instructed disciple of the noble ones — who has regard for noble ones, is well-versed & disciplined in their Dhamma; who has regard for men of integrity, is well-versed & disciplined in their Dhamma — discerns what ideas are fit for attention and what ideas are unfit for attention. This being so, he does not attend to ideas unfit for attention and attends [instead] to ideas fit for attention.

All the Fermentations

Basically it helps to have a bit of humour here, to be more relaxed about this, to realize that there are things that we don’t know answers to.