They're most likely animals adapted to life in the ozone layer, there is nothing inside them besides whatever they have as blood substitute and organs, they are something like super fast, armored floating snails/jellyfish, we still never caught and don't understand, just like deep-sea ocean species, but also there are still places left not a single step onto its grounds. Most of the US-Americans forests, all the woodlands of Russia, and big parts of the South American and African rain forest. As well as the Sahara and most other heavily deserted regions. Perhaps "UFOs" have a larvae stage that can only grow up on solid ground.
