The most common and most likely answer to this crossword clue is the 4 plus 6 letter word DARK MATTER
I've seen this crossword clue in Financial Times
D
A
R
K
M
A
T
T
E
R
More clues leading to the result DARK MATTER
- Evil affair — stuff we can't see
- It’s not seen as a theoretical universal presence
- Market trade almost exploded – it’s conjectured to be about 90% of everything
- Day before vessel problem is an elusive major universal influence
- Gloomier when getting to grips with dull cosmological hypothesis
- Secret affair that may be quite influential
- Sinister business that’s apparent throughout the universe
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The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.