Do aliens really exist?

In a series for the Discovery Channel, Professor Hawking said it was perfectly rational to believe that intelligent life exists elsewhere. He said "The real challenge is to work out what aliens might actually be like."

Personally, I think this is a very difficult and somewhat chilling topic. The idea of extraterrestrial life scares me a lot.On the one hand, it's easy to think that the Earth is not the only planet that has intelligent life. The question is whether there are: tens, millions or billions of aliens, or if they are much more intelligent, and therefore, they could dominate us. Anyway, I think its a bad idea trying to contact other forms of life. We've been trying to kill our own species for thousands of years. What if they decide to do the same with us?