It was mentioned in the movie Guardians of the Galaxy: Vol. 1Guardians of the Galaxy: Vol. 1 as well as in this articlethis article and confirmed herehere, that:
Knowhere — the mining colony located on the edge of the universe in the decapitated head of a long-dead Celestial beingCelestial being
[Links added by me]
Which Celestial Being did the head belong to? (if known).
I have also checked the Wikipedia page for KnowhereWikipedia page for Knowhere, but the closest thing to its origins was this line from the creators (Dan Abbnett and Andy Lanning):
Honestly, they just came to us. The severed Celestial head was, I think, something that popped out of Andy's mind one day. Similarly, one morning, I said "what about a talking Russian dog?" We run with these things and develop them together. It's hard to pin down where exactly they originate.