Go read some of the forbidden archeology genre. There has been stuff found hat appears to me mechanical or electrical embedded on rock strata where it shouldn’t be. And because they were found where they weren’t expected...they were dismissed as unexplained.
I agree that the vast distances make space travel improbable. I think a vast majority of UFO activity is earth based military stuff. But I would not dismiss something dimensional or something along those lines.
But not impossible. Especially for "ark" ships. We are on the threshold of being able to identify, not just "goldilocks zone" planets, but within the next ten years, Earth-like ones. If civilization survives obama/agenda 21, there's a strong likelihood that artificial intelligence and robotics will have advanced enough that they will be able to replicate themselves and, if interested, take DNA samples with them in order to spread out into the universe seeding all the nearby habitable or terraformable planets within two hundred light years within the timespan of a few thousand years - which is nothing to self replicating AI, and watching and learning from evolution as it unfolds, both carbon and silicon based.
It could have already been taking place these past billion + years.