Space

I’m not sure why that’s a conceptual hurdle. Electromagnetic radiation, including the visible light spectrum, is one of the primary methods in which we gather data about and interpret the universe. To say that the matter is “dark” is to say that it’s not detectable on the electromagnetic spectrum to us as we know it.

It’s not an uncommon turn of phrase, it’s the same reasoning for the colloquial term “going dark” regarding radio communication silence.

To say that it’s “invisible” or “clear” would imply the existence of some property causing it to be so. This would also imply the presence of interpretable data in order to term it as such, when in truth none exists. You could perhaps say “unknown” but then that’s truly arbitrary, “dark” at least implies the opposite of “light”, i.e. detectable and serves a conjectural purpose in that sense.