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