It was mentioned in the movie Guardians of the Galaxy as well as in this article and confirmed here, that:

Knowhere — the mining colony located on the edge of the universe in the decapitated head of a long-dead Celestial being
^{[Links added by me]}

Which Celestial Being did the head belong to? (if known).

I have also checked the Wikipedia 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.