i dont doubt that out of the tons of planets and solar systems theres at least one scenario where life on one planet has encountered life on another planet, thing is, the chance that there is a planet with intelligent life near enough for you to reach is so small that the chance of both of the species being the same level of technology would be improbable, it would be storm troopers verse ewoks, and it wouldnt go the way it went in the movie. or the way it goes in avatar,the more advanced race wouldnt even need to send out ground troops, they could just send bombs from orbit, no chance of survival.

honestly, if another race can manage to find and get to us, they are obviously far superior to us, and we would barely have a chance, i doubt we even have any space defenses that could even help.

and even if by some chance the other species had no intention of harming us, we would end up doomed, they wont just come down, "hey, here is some super advanced technology for free, we just wasted all of our resources to come give you this stuff and check you out, kthxbai!", i mean, humans hate eachother because of skin color, and because of religion, can you imagine how quickly they would turn on aliens with the negative connotations we have of them?

we would just be exploited, like the conquistadors exploited the south american natives,how english exploited native american "indians", and how everyone exploited africans, hell, rome exploited everyone,

so now how in the world could you possibly even believe that a alien race could have visited us and not done anything?

i can say without a doubt, there is life out there, and if they visit us, we will know.