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Big gap in astronomy (5,4)

Ross

I believe the answer is:

black hole

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'astronomy' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'big gap' is the wordplay.
'big' becomes 'black' (I can't justify this - if you can you should believe this answer much more).
'gap' becomes 'hole' (hole is a kind of gap).
'black'+'hole'='BLACK HOLE'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for black hole that I've seen before include "discovered in Calcutta" , "Astronomical curiosity" , "Space left after collapse of a star" , "object in 21 [SPACE] having very strong 20 [GRAVITY]" , "'Region of space from which nothing can escape (5,4)'" .)

I've seen this clue in The Mirror.
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