There's quite a bit we don't know, but what we DO know is that we're using pretty much all of our brain pretty much all the time. There are variations in the degree to which we're utilizing each part at a given time, but every part has a (known or hypothesized) function and is active. The way the brain works on a fundamental level means that the idea of only using some amount of it, as if there were a slider you adjust up and down somewhere, is complete crap. There are so many other ways they could have MacGuffined in the pretense to making Lucy a superhero that would have worked better. 

Usually, I can get past that sort of stuff, but the brain thing really pisses me off. 

ANYWAY, did you read my damn story yet?