You could also say that women who are not married by 30 have other priorities and marriage isn’t one of them. There seems to be a saturation point for each generation after which the uptick slows to a trickle. You could make the argument that fewer women in each successive generation are making marriage a “must” in life.
I would bet you this data would be inverted for women with a college degree by given age, i.e. younger generations are achieving higher levels of education by 30, but it likely levels off gradually as well since not everyone attends college.
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.