Actually, I think the universe is filled with different kinds of alien species, but I tend to doubt that they are here observing us.

Space is so huge that is must be very hard to get around, even for very advanced aliens (our current physics says that is would take thousands of years and huge amounts of energy to get to even the closest stars). And there are so many stars in the galaxy that it seems unlikely they would go through all that effort to come here. And on top of that, the earth is pretty old, so it seems unlkely that if aliens did come to earth, they would just coincidently have picked NOW to do it, as opposed to a thousand years ago, or a million, or a billion.

You can't prove that aliens have not come to earth to observe us, and I kind of hope that they are here, but I think it is not very likely.