Oh! I get it!
See, "Next Up, Goku" is the title of an episode from the
TV anime, not the title of a chapter from the
original series. In the
original series, the Supreme Kai never even spoke of the Dark Star.
Also, a screenshot from the Dragon Ball Wiki? Really? Is this quote even from the original Japanese dialogue or the English dub?
No, Daizenshuu 4
clarifies that the Dark Star merely lacks a sun, as opposed to existing "in the far end of the universe, where no light can reach it".
Hey, if the Supreme Kai referred to its location as "the far end of the universe", why is this being considered part of the unobservable universe?