"Other theories are big bang universes are created when parallel membrane universes touch for an instant"
That's probably possible Dann if that other universe is an anti matter. We are all made up of atoms with a positive charged proton and negative electorns. The reverse of that could cause a heck of a big bang if they ever touched.
You right that we are only here for a visit. A relative short one at that but I thank my lucky stars every day for the time period we were born into. If there is karma, we sure must have done something good to have earned this trip. We live in the most prosperous era ever know to our kind and us as individuals in one of the best places on the planet to boot. Ice cores have shown that this temperate period between ice ages are the anomalies and the ice age is actually the norm. We won't see it but there will come an age when it's ice age again and any further growth of our species will be drastically reduced. It maybe have already happened before. This time around our brains were developed enough to really take advantage (between ice ages) and we have thrived. Nothing lasts though. All species will only support what is possible in a given circumstance and then dies off. We are in the Algae bloom of mankind right now. It can't and won't last. I'm only talking short term of course, on our planet. Here eventually the sun will lose mass and expand and probably torch everything on earth long before it burns out and earth is a frozen rock again. Our kind will never get anywhere else either since at the speed of light, there is nothing remotely close to us that we might populate. But maybe since we are all matter and a part of the stars/universe, what with the billions of galaxies out there containing zillions of stars and planets, there is bound to be other situations with the same tree of life that has happened here. One can only take comfort in the fact that somewhere out there life continues on.
That is untill the universe expands beyond the capability of gravity to hold things together. Dark matter, dark energy make up much more of the uni than the matter we can actually see and we are only just now begining to understand how that works. But thats another post.