Aliens exist, but I really really doubt they'd ever come to us.
Say a 10 lane super-highway is built next to an anthill. Those on the highway won't notice the anthill. Even if they did, they wouldn't stop to say high and share technology with them. The ants wouldn't know what the highway is, they have their own business to worry about.
Now replace the anthill with Earth and the ants with humans. Why would an alien race that is advanced enough to travel the needed distances quickly stop on Earth? They wouldn't, just like we dont' stop by anthills beside the highway.
So aliens do exist, so more advanced than use, some less advanced, some sentient and others not. But they won't stop by for a friendly visit and share their technology like we seem to think. If they do come, it will either be via a crash or for conquest.