Simply has to exist in numbers, in deep space (5)
I believe the answer is:
chasm
'deep space' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for chasm that I've seen before include "Abyss" , "One yawns" , "A deep opening in the earth" , "Deep fissure; gulf" , "One gapes" .)