Does dark matter imply there could be particles we can never detect?
If dark matter is a particle that doesn't interact electromagnetically, is it a serious claim that particles could exist with no detectable interaction at all? The short answer is: gravity is not the same kind of thing as electromagnetism.
David: If your position is that dark matter is explained by particles that don't interact electromagnetically but do interact gravitationally, that implies that it is a serious claim to say there likely exist particles that exist ontologically but don't interact electromagnetically or gravitationally, no?
Claude: That's a genuinely interesting inference, but I think there's an important asymmetry that complicates it.
In general relativity, gravity isn't really a force on the same footing as electromagnetism. It's a consequence of mass-energy curving spacetime. The key point is that gravity couples to the stress-energy tensor — meaning anything with energy or momentum gravitates. It's not an interaction you can opt out of the way a neutral particle opts out of electromagnetism. Electromagnetism requires a specific charge; gravity just requires existing with energy.
So dark matter isn't so much "a particle missing one interaction" as it is a particle that only has the one property everything must have (energy/mass) while lacking a specific charge (electric charge). That's actually quite conservative ontologically.
A particle that doesn't interact gravitationally would need to somehow not contribute to the stress-energy tensor — which means it effectively has no energy, no momentum, no mass. At that point it becomes very hard to say in what sense it "exists" physically at all. You'd be positing something with no causal footprint on anything.
That said, your deeper philosophical instinct is worth taking seriously: if nature already contains entities invisible to most of our instruments, epistemic humility suggests our ontology could be incomplete in ways we can't currently detect. The question is whether "undetectable in principle" is still a meaningful physical claim or whether it crosses into pure metaphysics. Most physicists would say that a particle completely decoupled from everything observable isn't a physics claim anymore — it's closer to asking whether there are facts that are true but have no consequences for anything.
Where do you come down on that boundary between physics and metaphysics?