Not metaphors . We know these are out there — we just don’t know a lot about them. The situation is similar to atoms in second half of 19th century : we knew they are there , we knew some of their properties but only it the first quarter of 20th century we learned how much more there is to learn about atoms
Metaphor probably isn't the right word, but it's not really wrong either. "Placeholder" is also sorta correct but not entirely. Dark matter is a placeholder for an as-yet-unknown thing that interacts with gravity, in the most popular theories. There are less-popular ideas--still given serious study and consideration--like MOND that may some day explain the effects currently labeled as "dark matter". I don't think it's accurate to say "we know these are out there", and if it turns out that MOND or some other alternative explains observations, "dark matter" will turn out to have been fairly metaphorical (or just plain wrong).
We say "dark matter" rather than "weird gravity behavior" because the evidence thus far doesn't just look like gravity being weird. It looks like stuff, actual honest to God stuff, floating around. If MOND is right dark matter does not exist and never did. Dark matter is the name of the actual matter we believe is out there; we could be wrong, but it's not just a generic placeholder.
Atoms is a good example. Are atoms really out there in the way that most people would imagine? ("Electrons orbiting a nucleus of protons and nucleus") Or are they all actually fields of statistical probability or some other concept that is basically impossible to imagine in a physical world but matches our equations?
They're "actually" what QFT says they are, but the rest isn't wrong either. It's not wrong to say "a house exists" because it's actually a pile of bricks in the shape of a house.
Dark matter must be "matter" if it exists, because there's not a category of things that aren't matter.
> Dark matter must be "matter" if it exists, because there's not a category of things that aren't matter
I mean, ehhh? Like, yes, absolutely, no matter how weird dark matter turns out to be it's the obligation of our definition of matter to adapt to it, but it's also not the same kind of category error as saying atoms don't exist. Depending on how weird gravity's integration into QFT is, dark matter could be arbitrarily divorced from what we expect matter to behave like. If, horror of horrors, there is no such integration, I'd say it's fair to call dark matter something truly other.
Sure, I guess what I mean is it'd still end up in on the table of subatomic particles whether or not it's a new one.
Maybe if it's something like another universe of stuff that only interacts with ours through gravity then we'd just be unable to find out which kind of stuff it is? But I don't know how we'd even get to that point.
