Other worlds exist in the sense that there is so much humans don't understand.
At one time, looking through a microscope was to look into another world. Then we learned of bacteria, then molecules, then atoms, and now we are beginning to understand quantums but they are very mysterious.
100 years ago, flight was new. 50 years ago, the Moon was another world. We are still discovering new worlds today but the universe is vast and there is still so much we don't understand.
All that said, I reject the idea that man of the past knew of worlds that we don't know of today. It is implausible. We are in a vastly better position today to understand these worlds than mankind has ever been.