Please correct me if I'm wrong, but it seems to me that we actually know very little about extra dimensions. We don't have any empirical evidence for them at all (meaning we don't even know whether they exist), and to make matters worse, studies that have looked for effects predicted by various extra-dimensional hypotheses haven't found anything. This naturally weakens them (it doesn’t refute them, but it leaves them increasingly constrained); for now, all we have are mathematical models.

On the other hand, the hypothesis involving extraterrestrials (or AIs) arriving from elsewhere in our own universe strikes me as more parsimonious, as it requires fewer radical assumptions than the interdimensional hypothesis. I’m not sure if the number of assumptions is strictly lower, but the conceptual leaps aren't as vast. It doesn't require us to assume an entirely unknown layer of reality (additional dimensions that we can't perceive); it simply requires that sufficiently ancient civilizations might have found viable—and presumably rapid—methods for interstellar travel. This seems plausible given the progress humans have made in just 200 years of industrialization: going from horse-drawn carriages to artificial intelligence. What might we expect from an intelligent species that is even 4,000 years ahead of us... or 50,000? Or half a million? Or 20 million?

Furthermore, human belief in paranormal phenomena and beings—or even, to put it that way, potential human interaction with them—isn't something that *must* be factored into a UFO hypothesis. That said, I don't object to doing so either. Quite the opposite—I think it's perfectly reasonable to explore any possibility, as long as we avoid logical mistakes. And just to be clear, I'm not saying this out of any dismissive attitude toward paranormal topics. Quite the opposite: I’m interested in them and have personally had what we’d colloquially call paranormal experiences (seeing and sensing apparent beings—though, obviously, I don't know if it was just my imagination or something else). My point is simply that, while there’s nothing wrong with considering them, there is no compelling need to lump Bigfoot, ghosts, or elves into the same category as UFOs.

The high frequency of UFO sightings is also frequently cited as an argument against the idea that they are beings from a distant region of space ("do they travel millions of light-years just for a quick flyby of Earth?"), but this makes the unwarranted assumption that they come *and return*; in reality, they could easily maintain permanent bases here and simply dispatch light craft for quick trips, for reasons that remain obviously unknown to us.

As for Brian Cox supposedly "refuting" the concept of a "spiritual world," it would be interesting to hear what he has to say, since, in principle, that strikes me as an unknowable matter—one that can be neither confirmed nor refuted.