Time travel, as we understand Physics, is impossible. Until there's some form of scientific breakthrough that proves moving back in time is possible, I'm not going to start theorizing about time travelers and ancient aliens.

It's more than likely that intelligent life does exist somewhere out there, but as you said: the universe is vast. Even if there is another civilization with our exact level of technology, it could very well be on the other side of the galaxy, let alone the universe. It takes light a hundred thousand years to get across the Milky Way. Unless somebody sent out a radio signal in our direction a [i]long[/i] time ago, we're not hearing anything from anyone for a very long time.

I don't think aliens have ever been to earth. Hell, if they're out there, I doubt they even know we exist. It just isn't probable, and it flies in the face of everything we know about space travel.