So, I'll "complete the thought."
Once the stated hypothesis is strongly supported, and even though we do not have evidence of the next step in a hypothetical train (i.e. if not made by us, then what is the Causal Agency?), we, because we cannot resist doing so, create a list of imagined speculations which we label "alternative hypotheses" for the causal agency.
Now we KNOW that we don't have much if any evidence to distinguish between these guesses, but it of course turns out that the answer to this is ALL that the general public is interested in. So ALL media coverage and popular culture response strongly emphasizes the part of this which we, as scientific explorers, cannot answer for them. This leaves the public arena WIDE open for liars, snake oil salesmen, and every form of money and/or fame craving scumball. These people quickly become the "face of the field."
The structure of US "culture" and the general profound ignorance and impatience of most of it, allow persons with an agenda (not concerned with Truth) to simply repeat over and over what they want to manipulate "you" with, and after a hundred repetitions a fair number of persons believe anything --- see politics as a second such phenomenon. The phonies on Ancient Aliens are currently the primary public offenders in the UFO arena, as , astoundingly, almost zero content in their presentations is intellectually honest and everything is deliberately heavily slanted. With scam masters like them in control, the UFO research community has almost no chance to clarify things.
Nevertheless, real UFO researchers are asked for their/our opinions about the Causal Agency regularly, so we have to think about our responses to that. Because an honest response isn't simple and quick, all of our responses are ineffective. Mine goes sort of like this:
A. Folks have tried to come up with theories about what is behind the UFO phenomenon since its popular beginnings in the late 1940s. Here are some of those theories: 1. Angels; 2. Devils; 3. Daemons (paranormals not so demonic); 4. Atlantaeans; 5. Masters from Shamballa; 6. General underworld creatures; 7. Cryptozoological things; 8. Intelligent Lightforms; 9. Time Travelers from our future; 10. Beings from a parallel reality alongside us; 11. Extraterrestrials from distant advanced worlds; 12. Extraterrestrials from Mars (or some other Solar System location); 13. Psychic Thoughtforms created by us; 14. Psychic Forms generated by the Earth Consciousness GAIA (trying to get us off destroying it.) ... etc ... as you can intuit this is only limited by your own imagination linked with your sophistication in folklore knowledge.
B. People have jumped on different ones of these. Conservative Christian ministers immediately went to "Devils/Demons", but Billy Graham went for "Angels." Even holding onto one's shred of scientific approach, some of these just seem very unlikely. But note one big thing: NONE of these ideas is "normal." ANY of these would shake up everyone's concept of reality if generally "approved." This is why almost everyone who really thinks about this is a bit afraid of the answer.
C. Maybe the most brilliant scientist who really dedicated a lot of his life to investigating UFOs (Dr. James McDonald, atmospheric physicist of the University of Arizona during the 1950-60s and through the Condon Study era) had an answer that I still believe is best. All of such hypotheses could, under stretched imagination of either small or colossal proportions, be presented as a cause for what we see in ufology, but most would be extremely weak and unconvincing at any level. BUT, The Extraterrestrial Hypothesis (#11 above) is THE LEAST INSUFFICIENT of the whole. In fact, when one considers what we think that is true of the universe and our galaxy is true (as presented as normal science in textbooks), then this hypothesis violates nothing in the currently accepted probable reality.
D. Once the original research hypothesis is strongly supported (as it is), THEN one finds oneself led to the ETH as the strongest possibility even though we cannot prove it.
This is, I believe, where most of the world's best actual UFO researchers stand.