I believe some form of life exists out there. The universe is so big, and for us to think we are the only life, would be pretty small minded wouldn't it? Now, if you're asking if the green guys, with big black bug eyes from movies really exist, I'm going to go with no.
We don't know but it's possible I believe they do exist There are about 300 BILLION stars in the Milky Way Galaxy alone, every day a new planet Is found we think we know a lot but the truth is that we don't ..
Have you seen Prometheus? Check the storyline. The basis of the scientist discoveries that prompted the voyage is not far from historically correct. Spooky to think we might be the experiment of some strange civilization.
Now I don't know why but the definition of alien are technically strangers, which doesn't have to be from another species. If you want aliens in the US, check out Mexico. Now cj's answer is optimistic but life is a really complex sequence with rigid requirements that you have to hit in the right order. Since we've discovered more planets in space, the orbits of those planets are huge, have oval orbits toward their stars and are located far, far, away.