Of course, all rational and intelligent people understand that everything which exists has already been discovered. If there really were aliens, there would be some concrete evidence that they exist. Apart from all the evidence, eyewitness testimony, video, statistical probability, and historical record, there is no evidence whatsoever that other sentient beings exist in the universe. Furthermore, it should be apparent that any form of inquiry into this topic is prima facia evidence of insanity. Any experience that you have which does not conform to the general consensus of what does or does not exist should be treated as an abberation and you should immediately see a psychiatrist if it repeats. If, for instance, you drop a 1lb ball and a 5lb ball of the same size and they hit the ground at the same time, please see a physician immediately as everyone knows that heavier things fall faster than lighter things. If you see a rainbow after splitting white light in a prism, this is also an abberant event in your consciousness and should be treated with the utmost concern.
Personally I believe aliens exist and have been visiting the Earth for some time, perhaps into the thousands of years. But they aren't exactly physical beings you can poke with a stick. They have an ephemeral quality, able to slide in and out of our perceptual field. There is a certain malleability inherent in their appearances, and I think that Djinn, angels, faeries, aliens, and "gods" are probably all manifestations of the same "species" of non-human intelligence. Like some of the finer effects of quantum physics, they seem to partake of a different form of reality than rocks or cars or asphalt. But of course, such an inquiry should be shouted down as loudly and often as necessary. Any investigation of what is not already known is an insult to the hardworking scientists who already know they know everything.