Yes I do.
Technically, aliens do exist. An alien is defined as
1. A person who comes from a foreign*country; someone who does not owe allegiance to your country
2. Anyone who does not belong in the environment in which they are found
3. A form of life assumed to exist outside the Earth or its atmosphere
Considering that the first two exist, aliens do exist.
You must be referring to the 3rd meaning and even then I believe they exist.
Now this is not proving anything, just a case of probability.
There are approximately, say, 180 billion galaxies in the universe. Now, the average stars of each galaxy are, let's say, 300 billion(this must be the minimum I guess). Now per star, there is a average of 1.5 planets.
What are the chances that of so many planets, Earth is the only one with life?
PS - I didn't even start on Nazca lines, crop circles or the Nevada incident.