a) alien life existing
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c) how would we interact
d) do you think we're being buzzed by real aliens
I'm going to do this first... ahem.
a) Well, the galaxy is extremely large so I think alien life would exist (it exists on planets in our solar system already) but I think intelligent alien life would also exist. For instance, humans aren't self-aware except by the most loose definition of the word, so there would be other 'intelligent animals' all over the place... The reason we haven't been contacted is, I think, because space travel over any large degree is impossible.
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c) Total war. There would be no peace, even among almost-humanoids, because they know that you know that they might attack and destroy you, and you know that they know that you know that you might sneak an attack and destroy them... Between Jupiter/Earth life this would probably be more profound.
d) No. But maybe we were a few thousand years ago...