Full Idea
That 'there might have been a dozen more fundamental particles' is true, but not appropriately about any existing entities or their properties. Since Truthmaker says that all truths are about existing entities, it must be false.
Gist of Idea
Speculations about non-existent things are not about existent things, so Truthmaker is false
Source
Trenton Merricks (Truth and Ontology [2007], 6.VI)
Book Reference
Merricks,Trenton: 'Truth and Ontology' [OUP 2007], p.145
A Reaction
Since I don't necessarily agree that 'there might have been a dozen more fundamental particles' (see Scientific Essentialism), and I take the disagreement to have some basis, I doubt this idea. What stops 'there could be circular squares' from being true?