Oh cool! A real science thread! I've actually looked into this, mostly cause I don't like living here, but partly because it's just a cool subject.
First of all, life surely exists somewhere else out there. People just need to stop thinking they're humanoids. Everyone seems to imagine alien planets resemble ours (Even scientists). Our planet has an incredible variety of ecosystems that allow a huge number of life forms to live and procreate. Odds are, planets in our solar system have similar ecosystems capable of sustaining life. Mars for example has a small chance or sustaining life. Not exactly human life, but bacterial, viral or archeabacterial. (I think that's the word, I know it from French so I just loosely translated Archeobacterie to ^ that) There are also a few other moons in our solar system (Too far from the sun of course) but with possible chances to sustain life if we could pull them closer to us. It's not only about proximity to the sun, we have to factor in gravity if we're thinking of putting humans on other planets.
Now, considering time travel, the speed of light, and the size of the universe, it's nearly, and practically impossible for another life form to reach us. Not only because it would take them billions of years just to get here, but because we would notice them far before they ever got to us. If they traveled through space, they would have to dodge debris, planets, stars and so much more. They would also need to figure out a way to store and manage food and fuel for their ships and passengers. All life forms need some sort of nourishment, and you can't move something without a force of some sorts. So for them to be able to travel, they'd for sure need a fuel, and for them to survive, they'd for sure need a food source. Considering this, and considering the current "Earth" like planets we've discovered, there's no way an alien life form would consider coming to Earth to try and farm our resources if they have enough to make it all the way here. I also doubt they'd care about us if they were looking to conquer and dominate the universe. We're so pathetic and puny, nothing would want to bother with us.
Finally, back to the speed of light theory. If there was however a race of beings that could withstand the power of the speed of light, they could travel between planets a little bit faster. Remember, it takes 8 minutes just for the light of the sun to reach our planet. Our sun isn't really far from us at all in consideration to the other stars. I think it'd be fun to travel through time and space, but our bodies just can't handle it. Until someone figures out how to completely reconstruct a human body at the speed of light, we'll probably never chance a maned universal exploration mission.
P.S. I personally hope aliens look like underdeveloped monkeys. Like, maybe have no eyes, just rely on their sense of smell.. Cause you can really go a long way just off smell.
