I'm so taken aback by volume of the Universe (6)
I believe the answer is:
cosmic
'of the universe' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cosmic that I've seen before include "Johann Sebastian --" , "Great" , "Enormous" , "To do with the whole universe" , "Significant" .)