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They're both "dark" because neither interacts with electromagnetic radiation.

I share the opinion that they're names for holes, not entities; but I'm not qualified to defend that opinion.

That's true but even more than that, we're not really sure dark matter is actual particles at all. Modified gravity for example is one alternative explanation that don't require a new particle or field.

So it is something but we don't know what, right? Is it safe to say it's not something from the periodic table or we simply don't know at all?

And, in order to understand a something, do we always need that something to interact with EM radiation?

Neither "dark matter" nor "dark energy" can be made from atoms (i.e. from the periodic table), because atoms interact with photons. I think to say "it is something" is a strong claim, requiring strong evidence.

Dark energy, in particular, can't be a kind of "stuff".

We could "understand" something that doesn't interact with EM; but we'd have trouble observing it.

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