1) No unassailable evidence exists that we've been visited by otherworldly beings. None. Such evidence could conceivably exist in the form of alien bodies/biological remains/samples (for which a host of scientific analyses could be conducted, for example genetic or protein chirality tests), fossils, metals or other materials from spacecraft or their fuel, radioactive decay products, the spacecraft themselves, or gravitational anomalies in the solar system detected by LIGO. Presenting and making available something from these categories to the scientific community would likely decisively resolve the issue in favor of an intelligent non-human presence on Earth. It is highly significant that no such evidence has ever been demonstrably forthcoming.
2) There are too many ways in which human testimony can become distorted through confabulation, embellishment, hallucination/altered states of consciousness, mass hysteria/mass psychogenic illness (often precipitated by media stories or anecdotal observations), or outright lying, fabrication and/or hoaxing (people can also come to believe aspects of their own lies. We should never underestimate the role of self-deception in the annals of "ufology"). As decades of research have clearly demonstrated, human memory is highly editable, eyewitness testimony of supposed extraordinary phenomena is of questionable value at best, and perception at the time of a supposed event is highly subject to the witness's psychological and emotional state. Memories can influence current perceptions and behavior, and individual perception within a group is subject to influence and “contamination” by peers. Events can be interpreted through the lens of themes and motifs perpetuated through the media and absorbed by the witnesses beforehand, often unwittingly. Without extraordinary evidence for extraordinary claims, ET/other-worldly explanations are superfluous given what we know about human psychology. There have been many cases in which witnesses were adamant that they saw something extraordinary, only for prosaic phenomena to be exactly matched to the time, location and/or bearing of the sighting; if people can misidentify prosaic aerial or astronomical phenomena so dramatically, then it seems highly likely that this can account for currently unsolved cases as well. Reinforcing this point is that witnesses to the same event often present wildly divergent descriptions of it, which again underscores how pliable and suggestible human perception and memory can be.
3) Unexplained does not mean unexplainable; more likely, it simply means that we lack the data to come to a definitive determination of what actually happened in any particular case. Often, we lack sufficient data to resolve particular cases because that data, which would be pivotal to solving it, has been lost or is simply unavailable due to the fleeting nature of the event. A dramatic case is provided by the Betty and Barney Hill case, in which the Hills reported being chased by a light as they were driving down the road. Years later, it transpired that there had been a watchtower nearby that was fitted with a powerful search light. Such instances remind us that just because data for something prosaic does not currently exist does not mean that it never existed.
In the absence of unassailable evidence, we should not ascribe unexplained cases to extraordinary occurrences, but rather to what we demonstrably do have evidence for from routine, scientifically verifiable experience from a host of studies, experiments and actual cases - that is, prosaic explanations interacting with fallible human perception. Until we do have evidence of such a quality that it can demonstrate that something extraordinary has occurred, we should fall back on our baseline assumption that what is being seen is not other-worldly.
It is notable that even with the Wikileaks mass-leak of government documents, not a single one shows any confirmation of government possession of alien artefacts/bodies, let alone a reverse engineering program to exploit alien technology. This would be suspicious if there were such a program, since it would surely garner a huge amount of funding and institutional support given the enormous geostrategic and economic payoffs that could be had from successfully exploiting extraterrestrial/otherworldly tech. I would hazard to say that such a program would at least match the Manhattan Project in terms of investment of human resources, money and facilities. Of course, the Manhattan Project did do a good job of keeping itself hidden from the American people and the Japanese during WW2 - but not, notably, from Stalin's spies. Wikileaks, meanwhile, was an unauthorized release of documents made possibly by computer hacking and digital technology. We have seen that even something as sensitive as the National Security Agency's crown jewels of spyware and hacking tools have become public knowledge thanks to counter-hacking and active measures operations (whether by an adversary power or by disgruntled or alarmed insiders; for details, see Thomas Rid's excellent book "Active Measures - The Secret History of Disinformation and Political Warfare"). We can be sure that the Russian intelligence services would very likely have caught more than a glimpse of an American super-program of reverse engineering alien craft if it actually existed, yet, again, we see no unassailable indication of this (admittedly, if the Russians did think they were on the trail of such a program, it might turn out to be American disinformation designed to throw them off. The Bennewtiz affair may provide a template for thinking about such a possibility, should it transpire). Meanwhile, in UFO La-La Land, we are made to believe, by the likes of Bob Lazar and George Knapp, that there is a reverse engineering program but that it is a pissant little side-project of the military (but not to worry, because Soviet scientists were invited to participate! Sorry, but not bloody likely).
The UFO field is replete with fraud, pseudoscience, credulous acceptance of extraordinary claims, shoddy and even outright unethical practices (such as the continuing dubious reliance on regressive hypnosis to extract memories, including by people with no experience or accreditation as medical or psychological practitioners), contradictory narratives, and a general lack of skepticism (for a review of some of the more unsavory aspects of all this, please check out Jack Brewer's "The Grays Have Been Framed: Exploitation in the UFO Community"). Given the lack of filters to keep out bad science and dodgy claims, this acts as a magnifier/positive-feedback loop and pressure cooker for generating still more extraordinary claims, as people seek affirmation within groups of like-minded individuals who share supposedly similar amazing experiences or who just want to believe. There is an institutional imperative, or at least a strong tendency, within "ufology" to give the "consumer" what he/she wants at every turn (witness the unctuous YouTube comments that plead, "Please, MORE videos like this!"). Many researchers/ufologists are not motivated to look for disconfirming evidence that goes counter to extraordinary narratives. Disinformation/misinformation adds another layer, feeding into the web and shaping future perceptions/expectations of “witnesses” – the experiences of which are then fed into another round of “analysis” by people motivated to come to an affirmation of the extraordinary narrative:
Download chart here. Note that this chart probably vastly underestimates the complexity of the web of interactions at play. Note also that the most salient node, from the point of view of hard evidence - that is, the blue box labeled "Physical event triggering experiences/sighting/reports" - need play only a very minor role in the overall nexus of feedback loops. These other connections are more than capable of running autonomously, churning out and reinforcing questionable narratives to skew perceptions and expectations. In short, this scene is the antithesis of where one should expect to find solid, clean data.
(Incidentally, a thought occurred to me as I looked at this chart: that there might be a clue therein about the erratic movement often ascribed to UFOs, and whether this movement might be a cognitive construct representing the witness's implicit sense that they are embedded in a nexus of interactions, both internal and external, and that the witness recognizes that he/she is being constantly shaped and modified by experience. This in a way is the opposite of another possibility to explain the famously erratic movement of the UFO (I believe that I first encountered this in a work by Jacques Vallée , though he suggested that interdimensional beings were actively playing tricks on us to shape our perceptions): that it represents a disruption and challenge to our standard categories about the universe and our place and destiny within it, and is a manifestation of our yearning to break free from those categories in order to answer fundamental questions that we are anxious about (this is somewhat linked to the "trickster" aspect often present in supposed paranormal phenomena). I highly recommend David J. Halperin's extraordinary book "Intimate Alien: The Hidden Story of the UFO" for similar and far more developed Jungian interpretations of various aspects of the alien/UFO milieu)
There are formidable, perhaps insurmountable, problems with interstellar travel, though there may ultimately be ways of circumventing them. An elaborate scheme might involve something like the following: imagine not that the aliens are coming here from there as biological beings, but rather that our solar system was seeded in the distant past with a robotic factory/harvester that exploits the materials of the asteroid belt, or some other resource, to produce semi-autonomous drones that are under the command of the harvesting "brain". These mini-drones might be able to traverse the distances between their base of operations/point of manufacture and Earth, and these might be what we call UFOs. The drones might be piloted by, or integrated with, cyborg organisms that we report as "aliens". The original mother-ship/harvester/brain, on the other hand, might have taken eons to reach us from its star system. However, the energies necessitated by the maneuvers and other capabilities often attributed to UFOs return us to square one: they would surely leave a signature (such as radioactive decay products or gravitational effects) that would be apparent and eminently detectable (and if the admittedly sci-fi tier scheme I've posited for UFOs as drones of an overseeing harvester is true, why would the ufonauts appear in such a diversity of forms in witness reports? Again, we can posit schemes within schemes to explain away such anomalies, and build them up to any convoluted degree we please, but then we are in the realm of pure speculation and fantasy). Similar problems prevail for the interdimensional hypothesis. As for the demonic hypothesis, it surely finds more parsimonious resolution in the realms of psychological and cognitive science. Theists can appreciate just as well as atheists that human cognition is wired in a certain way and that experiences of UFOs need not defer to literal agents of Lucifer.
Apart from the hypotheses for UFOs being extraterrestrial, interdimensional or demonic, there is also the suggestion that they are the technological creations of a hidden or "breakaway" civilization residing in the Earth's interior or in the depths of the oceans (note that the "breakaway civilization" label can also denote a different idea involving a "secret space program" wherein "elites" are establishing bases on Mars to escape the future collapse of Earth's biosphere, leaving the masses on Earth to fend for themselves in the ensuing apocalyptic hell-scape. You might have guessed, rightfully, that I don't credit such notions, even though I have many genuine misgivings about Earth's political and economic leaders). The problems with this hypothesis are legion and are, if anything, even more formidable than the aforementioned options, with the exception of the demonic hypothesis (or at least, that's my atheist inclination). For one thing, there are no indications from extensive geological surveys of the ocean and sea floors, using high resolution radar mapping, that there are any exit and entry points for the routine egress and regress of advanced craft. Even if the craft are imagined to travel from only one or a few points in the Earth's crust (thus making these points harder to detect), this would arguably still leave some sort of pattern behind, such as a concentration of sightings around certain regions of the Earth's oceans or of radioactive isotopes deriving from the crafts' power sources (which are often reputed to involve esoteric nuclear physics - unless, of course, they're running on Vril or some other mystical nonsense) and would allow for some sort of triangulation of their locations. Luis Elizondo, who has hinted that he treats the breakaway civilization hypothesis as a serious candidate, has offered no pointers (as far as I know) about where in the oceans we should be focusing our search efforts. If he has such pointers in mind but cannot reveal them due to non-disclosure agreements, this brings into question his utility in Avi Lobe's Galileo Project, which he has joined, and which aims for a fully transparent and publicly available investigation into UFOs/UAPS that will not be subject to military censorship. I hesitate to use the rather repulsive-sounding corporate lingo, but where are Elizondo's deliverables on this score?
To sustain a civilization advanced enough to engage in the types of metallurgy, materials science, mining and manufacturing processes, and logistical operations needed to build and operate exotic craft, the denizens of this mysterious hidden world presumably require substantial thermal shielding (and, indeed, venting of their own thermal waste) from the Earth's geophysical processes to carry on day-to-day life - yet no indication of such cavernous structures is evident from the seismic analyses routinely conducted by geologists to study the Earth's interior. If the beings who operate the craft represent a very ancient divergence in the hominid tree, as some have suggested to account for the physiological characteristics ascribed to the iconic Gray aliens (which in the view of these advocates are not truly aliens since, in this scheme, they evolved on Earth), then there is certainly no known fossil evidence for such an evolutionary trajectory. Some have suggested that the beings might have evolved to live in the oceans as aquatic organisms; yet, again, we see no evidence in the fossil record to suggest this, nor is an aqueous environment conducive to building a civilization (metallurgy, for example, would be much more difficult, if not impossible, to routinely carry out under water, and metallurgy is almost certainly a prerequisite for advanced civilization anywhere in the universe).
The breakaway civilization hypothesis is reminiscent of and may simply be a mutation of Atlantean/Lumerian/Hyperborean/Hollow Earth mythology (with splashes and dabs of Theosophy and Lovecraft thrown in). I should mention that I get a distinct dude-bro vibe emanating from some of the advocates of the hypothesis, and I don't regard that as insignificant given the very pedestrian social nature and motivation for much of "ufology". I readily admit, however, that it is a lot of fun to think about a subterranean civilization possessing hyper-advanced craft that can run circles around US Navy Hornets. I'm certainly not going to deny that!