I do believe that aliens (in this case the intelligent ones) exist, but no, they haven't visited us. It's possible, but less likely.

Aaand Fermi paradox: "The apparent size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist. However, this hypothesis seems inconsistent with the lack of observational evidence to support it."

If there are many civilizations out there, there are also many reasons not to contact us. Or maybe Rare Earth hypothesis is right; there must be so many specific requirements of a planet to be able to sustain life.

If the universe is as big as scientists tell us, then it's a distinct possibility that there is life out there somewhere. It's just a matter of your own beliefs since there's no real evidence to support the theory.

There has to be other kinds of "humans" who discuss this every so often in their own kind of forums.
Sif earth came up with everything first?

it is impossible that we are alone in the universe, the earth is only a microscopic particle compared to the universe, I think that even if we have not yet found life in our solar system, they will exist dozens of solar system in the universe.
I think also the aliens have come and are still on earth but our governments we hide the truth.

We are no wonder in the universe. Aliens exist for sure but most of them are unlikely able of interstellar space travel/communicationand it's even more doubtful if they visited earth.
Life developed on earth under most different circumstances. All you need is a planet with moderate temperature and certain elements.