The problem with this sort of statement is that your use of "could" is qualitative, not quantitative. Things are only worth considering in proportion to how probable they are, and that probability can be quantitatively determined from Bayes' Theorem, based on the information we do have (the prior probability). Just saying "X could be true" is virtually meaningless, because what really matters is if X is likely to be true. In this case, the scenario you propose is extremely unlikely, because we understand not only the complete periodic table of elements up through element 118 (i.e. there are no gaps), but also the subatomic makeup of those elements in terms of electrons, protons, and neutrons, the latter two being made up of quarks and gluons. Given all of this information, the domain of possibility for some nebulous alternatives with "similar properties" is highly restricted.



That such a claim bothers you is a problem with you, not a problem with the claim. The claim is perfectly legitimate and in line with every other claim of scientific knowledge. There are standards for evidence that rule out reasonable doubt and alternatives, and the standards have been met in this case. Your suggestion of "according to what we do know, the most likely..." is what we mean when we say "we know" in any case of knowledge. For some reason, you feel more confident in other domains of evidence than in this one, but again, that's not the evidence's problem.