Translation:By now probably everyone knows.
'es' is the object: Everybody knows it. It is needed, because to know/wissen always refers to something you do actually know. Ex: - You smell! - I know. There, knowing refers to the fact stated in the first sentence. Any indefinite group, like jeder, alle, keiner, etc. need third person masculine singular, wissen is nominative.
Just to further clarify, since "es" can also be third person singular nominative and so could be interpreted as the subject (before you reach the last word), there are two reasons why seeing it that way won't make sense:
If "es" is the subject then "jeder" would have to be in the accusative, which it is not.
More importantly though (and why a native wouldn't stumble when they got to the last word) "weiß es" is a common enough phrase in which "es" is always the object. The native speaker would be expecting a subject. If he got to the end and say something in the accusative, he'd probably need to reread.
It is a pronoun, not a noun, in both English and German.