Travel quickly back and forth, discovering what's out there (4,5)
I believe the answer is:
deep space
'discovering what's out there' is the definition.
I can't tell whether this definition defines the answer.
'travel quickly back and forth' is the wordplay.
'travel quickly' becomes 'speed' (I've seen this before).
'back' says the letters should be written backwards.
'and' is a charade indicator (letters next to each other).
'forth' becomes 'pace' (I can't explain this - if you can you should believe this answer much more).
'speed' reversed gives 'deeps'.
'deeps'+'pace'='DEEP SPACE'
(Other definitions for deep space that I've seen before include "even devoid of 12 [THIN AIR]" , "Region beyond our atmosphere" , "void" .)