I'm taking an astrobiology class in college... "Life In The Universe"... Boy is it interesting.
Think of this:
The Earth is part of a small solar system within the Milky Way Galaxy. Just in THIS galaxy, there are about 100 BILLION stars, like our Sun. There are vast groupings of galaxies spread out through the universe... Containing BILLIONS of other stars.
It's so massive, how could we ever really know what's out there when we can't even fully explore our own solar system?
Think of this as well:
Light years is how long it takes light to reach Earth - Enabling us to actually see it.
That means, basically... If a star is 13.5 million light years away from us, we are actually viewing it the way it was 13.5 million years ago. We don't know what's going on in the present around that star. There could be life evolving as we speak!
My text book gives the example of trying to view the planets of a "nearby" star as: Being in Washington D.C. and trying to see marbles orbiting a grapefruit in California. LOL!!!
We truly don't know what's out there... Which is why I believe that there is some form of life out there. And the fact that all the elements needed for life are everywhere in the universe... If it happened here, who's to say it didn't happen in some other star system? ^_^
*I love this class!*