There is plenty we don't know, and even more that we will never know if we rely on the governments to tell us. I have to believe that there is other species about that are much more intelligent then where we are today in that department.
All a person would have to do to come to that conclusion would be to actually look at some of the things that have been built here on earth thousands if not hundreds of thousands of years ago like the pyramids.
There is no way in hell that the pyramids were built using the primitive tools we have been told that were used. The fact is that we couldn't reproduce those structures right now with the same accuracy as they were built with back long ago. That should be enough right there to at least say that the potential for some other beings to have been here previously exists, and who is to say that they haven't come back from time to time to check on what they had built and left?