Remember before they went to sky island? Those 4 were tryna catch 'em before they went? And also he met Luffy @ the bar and they were disagreeing on everything? Luffy got owned by the punk Spring Spring guy and walked out and got some motivation from Blackbeard... Then Luffy and Zoro caught vibe from Blackbeards crew like "Not him. Them." like Luffy and Zoro knew something...

well actually darkness (well Dark Energy anyways) and gravity make sense together. its compacted but i remember reading some scientist's (cant remember who it was) THEORY on what gravity is caused by: Put simply gravity is cause by the motion of dark energy. or something like that, but regardless dark energy seems to have a lot to do with gravity from what i have read.

now as for the nullifying part, ya my only guess is that people have an idea of darkness taking everything away, like the saying "in darkness there is nothing" (except in my case there is always a chair there for me to stub my toe on...) so ya but thats my guess for that part.....