Oumuamua was an alien probe sent to spy on us disguised as an asteroid/comet to confuse us knowing we'd be skeptical to say it was an alien craft due to us not having any knowledge of alien life.
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Not large. I've thought of a natural explanation: an asteroid or comet like object that was hit on one side with a natural high-energy event (there are a wide variety of natural possibilities) giving it an asymmetric albedo, and thus need not be so small or so oddly shaped.
My Bayesian prior for the existence of aliens is not large because I think the odds are high they would be very widespread and thus very visible (including via spectroscopy in at least some of the billions of galaxies we can see)
