I think you’re reading Popper uncharitably, and his view of what physicists do is about the same as yours. He really is arguing against arguing definitions. “What is matter?” is an ambiguous question: it can be understood as asking about a definition, “what do we understand by the word ‘matter’, exactly?”, and it can be understood as asking about the structure, “what are these things that we call matter really made of, how do they behave, what are their properties, etc.?”. The former, to Popper, is an essentialist question; the latter is not.
Your understanding of “essentialist questions” is not that of Popper; he wouldn’t agree with you, I’m sure, that “What is it about antimatter that makes it so rare?” is an essentialist question. “Essentialist” doesn’t mean, in his treatment, “having nothing to do with experimental data” (even though he was very concerned with the value of experimental data and would have disagreed with some of modern theoretical physics in that respect). A claim which turns out to be unfalsifiable is anathema to Popper, but it is not necessarily an “essentialist” claim.
I think you’re reading Popper uncharitably, and his view of what physicists do is about the same as yours. He really is arguing against arguing definitions. “What is matter?” is an ambiguous question: it can be understood as asking about a definition, “what do we understand by the word ‘matter’, exactly?”, and it can be understood as asking about the structure, “what are these things that we call matter really made of, how do they behave, what are their properties, etc.?”. The former, to Popper, is an essentialist question; the latter is not.
Your understanding of “essentialist questions” is not that of Popper; he wouldn’t agree with you, I’m sure, that “What is it about antimatter that makes it so rare?” is an essentialist question. “Essentialist” doesn’t mean, in his treatment, “having nothing to do with experimental data” (even though he was very concerned with the value of experimental data and would have disagreed with some of modern theoretical physics in that respect). A claim which turns out to be unfalsifiable is anathema to Popper, but it is not necessarily an “essentialist” claim.