Actually I presently entertain 4 theories, or combinations of such. 1) That the typical orange balls of light or plasma seen as UFOs could be some as yet unknown telluric yet living entity.
I've speculated occasionally about space animals. A somewhat more likely possibility for these is some unknown physical meteorological phenomenon akin to ball lightening.
The thing about this class of phenomenon is that oftentimes it's just lights in the sky. The cause could be just about any light source, which leaves open a lot of possibilities. Most likely there isn't any single cause for all of them.
2) that our planet has been and may still be being visited by aliens from another planet.
That's certainly one of the possibilities and it can't just be summarily dismissed. I don't assign it a tremendously high probability though.
Why?
Because I see the initial appearance of life as very fortuitous and unlikely by its nature. That's because even the simplest forms of life are already hugely complex. Researchers devote their entire careers just to studying the hugely technical details of DNA, how information is encoded in the genome and how transcription is controlled. We aren't likely to get something like that just by mixing chemicals together in a "soup". (The creationists are right about that.) I don't know how life originated (it's one of the larger remaining mysteries) but I'm inclined to see it as the result of a long process of chemical evolution that involved competition and selection among simple chemical replicators. It probably was a matter of a whole series of intrinsically unlikely events, added together and ultimately cascading.
That implies a number of things. First, assuming that life is the result of a whole series of fortuitous events that might not have happened, or might have happened in different order, life is apt to be rare. It isn't something that's going to pop up everywhere. Life might only exist on a tiny percentage of extrasolar planets (one in many thousands perhaps) and it's likely to be a needle-in-a-haystack, separated by long distances.
And second, it suggests that any alien life out there is probably going to be very different than Earth life, biochemically speaking. If life is the product of a long series of fortuitious events, then alien life is probably going to be the product of a different series, that happened to result in something functionally similar. So there isn't going to be any interbreeding between aliens and humans, no alien-human hybrids. Having sex with an alien will be less productive of fertile offspring than having sex with a pine tree, which at least has a similar DNA-based genome. Aliens won't have the same anatomy as humans, they won't eat the same foods and they might not like our atmosphere very much.
And there's what appears to be the fact that more than 80% of the history of life on Earth consisted entirely of single-celled life. If the Earth is in any way representative, that suggests that many alien planets on which something functionally analogous to life exists won't have produced anything resembling extraterrestrial space travelers. The appearance of multicellular life on Earth might have been another of those fortuitious events that might not have ever happened if things had unfolded differently.
Then there's the fact that no human beings existed for more than 99% of the time since the Cambrian explosion. Multicellular organisms existed, but they were worms, trilobites, fish, insects, ferns, amphibians, conifers, lizards, dinosaurs, flowering plants, birds and non-human mammals. Life appears to have appeared on Earth some 3.5 billion years ago, multicellular organisms about 500 million years ago, and anatomically modern humans only 130,000 years ago. So if Earth is anything to go by, even if large complex organisms even exist on alien planets, chances are they won't be intelligent in our human sense. The appearance of intelligent life appears to be another one of those fortuitous evolutionary events.
And the scientific revolution was only 400 years ago. Even if intelligent aliens exist out there, they might not be travelling in interstellar space. They might not have any idea that the dots in their sky are stars like their own.
Add all the probabilities together and it suggests that the likelihood of interstellar visitors is very low.
Of course, the counterargument is that the possibility-space that constitutes the unknown is unknowably large. Evolution seems to propel life to fill that possibility-space like a fractal. Just because what happened here on Earth has an infinitesimal chance of being repeated elsewhere, doesn't mean that astounding things can't have happened elsewhere, things that we can't even imagine because nothing like them ever happened here. I take this counterargument very seriously and give it a lot of weight. But just because amazing and unpredictable things are possible elsewhere, doesn't necessarily mean that those things are traveling here.
3) that there are beings visiting us from another dimension.
That one seems most likely if we take the 'contactee' reports literally and factually, which I don't. The humanoid aliens, the abduction and human-alien hybrid aspects of the myth suggest time-travelers to my science-fiction sensibility, or maybe visitors from some many-worlds alternate Earth. The anatomical similarities, their comfort in Earth conditions, and the whole idea that they can mate with us and produce fertile offspring, suggest that things like the 'greys' are in fact human beings. They would seem to be no more different from us than Neanderthals, and we could apparently interbreed with Neanderthals. It's possible to imagine that they are our evolved (or genetically engineered) descendants from the distant future.
It's easy to create a science-fiction framework around that. Time-travelers aren't supposed to alter the past since that would threaten to set history onto another course and endanger their own existence. But future humans (or post-humans, depending on taste) might know that our period experienced the UFO phenomenon, which actually records their own time-visits. So as long as they conform to the UFO pattern they aren't changing their own past, but making sure that it happens as recorded. But they can't reveal themselves directly or leave future artifacts behind.
It's sometimes asked, 'If time travel is possible, then where are all the time travelers?' This hypothesis would neatly answer that question.
I'm inclined to give it a fairly low probability, it's just too speculative and science-fictionish. And it's too dependent on the 'contactee' reports, which I don't believe. I like it more than the extraterrestrial hypothesis though.
4) That we are seeing the emergence of some new kind of collective consciousness on earth that mirrors the cultural tropes of the time they manifest in.
I'm inclined to think that the most likely explanation of the UFO phenomenon, or at least most of it (there may indeed be one or more signals mixed in with all the cultural noise), is that it's an example of contemporary popular mythmaking. What makes it new and unlike mythmaking in ancient and medieval times, is it's strong modernist form. It isn't magical mythmaking or religious mythmaking, it's "scientific" mythmaking. It takes the familiar trope of supernaturally portentious appearances in the sky, and reworks it in the form of spaceships. The mystery airships of the 19th century might arguably have been an interesting modernist forerunner, suggesting the existence of hidden super-scientists with impressive resources whose futuristic (to the Victorians) vehicles were visible above them.
I'm even willing to admit that these could be terrestrial beings that live under our oceans, as reports of USOs (unidentified submersible objects) certainly suggest.
Those are very interesting. I will have to depart from H.P. Lovecraft and say that it's unlikely that there is any advanced civilization at the bottom of the sea. Certainly not a technological civilization, since we would have already encountered them. (Submarine sonars are very sensitive.) But the science-fiction nut in me asks 'If you were an alien or a time traveler, and wanted to create a local base safe from the eyes of the planet's inhabitants, where better to put it'?
So yes, I remain open to many theories, but I will not resign myself to just labeling it "unknown" so I don't have to look into it anymore. The objects DO have an explanation, and are not just unknowable objects, and it is our duty as explorers of reality to find the one that best fits the facts of the phenomenon.
The problem that I see is jumping to conclusions. They are space-aliens, they just
have to be! Or at the very least they have to be something transcendent and world-changing that will turn all of human life on its head. But we don't really know that. We shouldn't be confusing our own speculations and desires for what we really know.
That doesn't mean that we can't form our speculative hypotheses, as long as we don't confuse them with knowledge. But what we really need are hypotheses that can somehow be tested, and they seem to be in very short supply.
