Do aliens exist?

The often regular topic in the media with a bunch of different "evidences." Let's see what is true and what is not. As usual, define the main points :)

1. Life on other planets. Our universe is incredibly huge and rapidly expanding. So statistically life on other planets is quite possible. The only question is it intelligent life and how far their planet from us.
2. Technologies. From a scientific point of view, it is very difficult to move through interstellar space. It requires a huge amounts of energy and resources. Still not found is it possible to move faster than the speed of light and do not contradict the Einstein theory of relativity. Anyway, even the speed of light is very low in cosmological scales. For example, needed 4,22 years to reach just the closest star on the speed of light.
3. Motivation. This follows from above. According to popular belief, the aliens are stealing cows, leave marks, experiment on human beings, just fly and do other minor activity. It becomes obvious, that the aliens would not have to spend a huge amount of resources and time for such purposes.
4. Traces. Officially, there no reliable evidences of alien's visit were found.

So, we can say that there no and never been aliens on the Earth. Their technologies must be beyond the bounds of science and the aliens must be so strange that they would use their power to steal cows only. There is a chance but it is directly proportional to the absurdity of their behavior and anti-scientific in general :)