@Arjuna,
Hi Arjuna!
Every thing is an example of 'not cow' with the exception of 'cows', of course.
But if we answered every question 'what is that?', it is 'not cow'. Then we wouldn't really be doing anybody's education any justice.
'Nobody' as an expression, certainly exists. It is 'nobody' the thing that does not. Nobody, I'm sure you'll agree, is not a thing, it is an expression used to convey the absence of a thing.
To sum up - Nobody is present never in the physical realm, for nobody has no properties. Nobody cannot prove that this is false or correct because nobody is incapable of proving anything.
Best wishes Arjuna!
Mark...