This is about missing baryonic matter, not "dark matter". What's not clear from this article is why we knew (or thought) that there was baryonic matter missing

This is about missing baryonic matter, not "dark matter". What's not clear from this article is why we knew (or thought) that there was baryonic matter missing in the first place. Wikipedia has more info under "Missing baryon problem"[1]... basically it's theorectical predictions of how much baryonic matter there should be given Big Bang nucleosynthesis and the observed cosmic microwave background. The article should have gone into that more, typical sloppy science journalism. [1] https://en.wikipedia.org/wiki/Missing_baryon_problem https://en.wikipedia.org/wiki/Missing_baryon_problem