There’s still a lot about the universe we don’t understand. Saying so doesn’t make me a fanciful kook; it makes me a scientific realist. The idea that what we understand about the laws of physics won’t change in the next five hundred years, however, strikes me as unbelievable. If history has proven one thing, I think it would have to be that we cannot accurately imagine the future.
As to the UFO question… I have insufficient evidence to come to a conclusion at this time.
The Egyptian pyramids, obviously… haven’t you ever seen Stargate? It’s obvious that the pyramids were built by malevolent aliens since they’re very old and we don’t know everything about them. 
When I imagine what an advanced civilization might do to explore the universe I extrapolate from the sattelites we send out today. Then I try to project existing technology into the future.
OK, a silver disk enters the atmosphere, flies around a while, then leaves. Is this an alien spacecraft? Probably not. Occam’s Razor basically. But is it possible that it is an alien spacecraft dipping into the atmosphere to take some readings and then move on to the next planet? Sure. It could be some kind of un-ah-aliened vehicle with sensors snooping around the solar system. Proving it is the rub, and the important part.
Flying actual aliens around is another story. For that I think you have to pack a lot of stuff. You know, the “mother ship” and all that sci-fi stuff. Or not. Who really knows?
Color me skeptical but interested. Now that we know how tough life really is, unlike when I learned science as a kid, the possibilities seem much greater that life is teeming throughout the universe. Problem is, based on the human example, intelligent life (and other life to a lesser degree) has a tendency to destroy itself, or be destroyed, over time. Good example, asteroids. We are now smart enough to know that given a period of millions of years the chances of an asteroid wiping out the human race is 100%. Nobody really disputes that. But, nobody in power takes the risk very seriously. Oh, not to mention toxic waste, nukes, biological engineering, etc…
I’ve seen plenty of cool stuff in the skies but there ain’t no flying saucers. I take the view that in this day and age there are few areas of the world without some form of surveillance - CCTV’s in towns, privately owned camcorders and cameras. If there were objects in the sky identifiable as alien space ships of some sort we would have positive proof of them by now. Simple as that really.