Much of the universe not seen to be important after nightfall (4,6)
I believe the answer is:
dark matter
'much of the universe not seen' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'be important after nightfall' is the wordplay.
'be important' becomes 'matter' (I've seen this before).
'after' says to put letters next to each other.
'nightfall' becomes 'dark' (I've seen this in another clue).
'matter' after 'dark' is 'DARK MATTER'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for dark matter that I've seen before include "A large proportion of the mass of the universe" , "In astronomy, theoretical non-luminous material" .)