With 60-Across, hypothetical substance in outer space ... and a literal hint to six black squares in this puzzle (4)

I believe the answer is:

dark

(Other definitions for dark that I've seen before include "With little or no light" , "Obscure - sinister" , "Sinister - evening" , "Overcast" , "Lacking light" .)

I've seen this clue in the Universal.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.