The concept that UFO’s represent mechanical or biological entities that traveled over the unimaginable distance of 100 light years to visit earth but never leave any confirmatory physical evidence is far fetched. However in a universe with “hundreds of billions of galaxies, each with billions of stars”, it is no so far fetched to beleive there are other, perhaps innumerable advanved civilizations out there, way way way out there. Problem is they are far too distant for them or us to physically visit each other. However remember earth has been broadcasting intelligible radio signals for 120 years. Good chance an advanced civilization or even a confederation of civilizations linked by radio communications that would make the Pony Express look fast, has heard us and is sending back some messages. Who knows they may be using earth’s atmosphere as a giant video screen. Perhaps some alien adolescents are having some fun with an advanced intergalactic video game.
Good thinking. Not saying that's what it is, but something is going on so we may as well think creatively.
In the Westall, Australia mass ufo sighting in 1966, several folks said the "craft" was solid at times and almost translucent at other times. It passed through a stand of trees that were so tightly grouped that nothing large and solid could have done so.