A lack of any convincing evidence, mainly. There have been countless thousands of reports of UFO’s over the last century, but most can be easily explained by natural phenomena or known human activity. For example, the sighting of a mysterious spinning spiral in the sky above Norway turned out to be a malfunctioning rocket spinning out of control. Other cases turn out to be the planet Venus (“There’s something hovering above the horizon, as bright as an aeroplane’s landing lights”), or flocks of birds catching the last rays of sunlight, or even the classic weather balloon.
Of course, there are some reports that have defied explanation, but it’s a huge leap to say “The experts don’t know what it was, therefore it must be aliens”. The number of explained sightings shows us just how easily people can misinterpret situations that are new to them. We also know just how absurdly huge the galaxy is, how many hundreds of millions of stars there are and how mind-bogglingly far apart they are. The odds of an alien civilisation even knowing we exist, let along having managed to send spaceships to visit, are extremely slim. It’s far more likely that we can’t explain some sightings because we don’t have enough information, or simply aren’t smart enough! Jumping to the unlikely-yet-most-exciting conclusion is fuzzy thinking.
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