It's entirely possible that Buddhism doesn't answer everything, just like science. While I've had some experiences and made various observations over the years that support the ideas of karma and rebirth I've still never tried to use it to explain everything about life on this planet (or others). But one thing the Buddha taught is that there are many worlds that beings can transmigrate to and from. Other planets maybe ? My vision is not that great so I don't have a perfect understanding of the universe and sentient beings within it. Science hasn't found life on other planets either so I end up with "I don't know" which is fine, because nobody else does either. I focus on the practical aspects of the Dharma, how it relates to us here and now.