the problem with UFO's and aliens is distance, space is freaking huge. Now if you want to discuss possible other life forms, other dimensions within our "space" is a real possibility, in fact a alien could be right in front of you studying you and you would never know it
One of the coolest and yet most depressing things about space for me is that life -- whether that's human life or other life -- within our local group will never be able to travel to other galaxies outside of using methods that are significantly faster than the speed of light. Sooner or later, anything outside our local group will be speeding away from us faster than the speed of light as the space between continues to expand. In the far distant future, there may be some life that looks up to the same spots we observed (such as the Hubble ultra-deep field photo) and see nothing but darkness; they may even believe their universe consists only of their galaxy or local group.