Well, no. We do use pretty much all our brain all the time. If you use a brain imaging paradigm like positron emission tomography, where you track blood flow throughout the brain, you see the whole brain lighting up all the time. You can use that diagnostically, because if a bit of the brain isn't lighting up, then that bit of brain is dead, basically.
The brain is a massively expensive piece of kit. In evolutionary terms, it's so unusual for an organism to carry round this massive ganglion of braincells that costs us so much of our fuel supplies. At rest, your brain's using about 20% of the available oxygen in your body, though it's just 1.5% of your body weight. The heart and the liver also punch above their weight, but the heart and the liver are actually keeping you alive, whereas you essentially don't need all that brain activity in order to be alive. So you don't have a brain without it working, constantly. What's using all that energy is it working all the time without stopping.
Part of this is that it needs to be ready all the time. If you think about the way information is stored in the brain, it's not stored in discrete little bits, it's stored in networks. Those networks need to be ready to be accessed constantly. They can't just be powered down, as it were.
And also, there's so much to do. If I'm sitting talking to someone, it might feel like all I'm using is my voice, but actually I'd have to control everything from my stomach up to my face for the act of speech itself. I'll be moving my hands around too, my eyes are constantly moving and focusing, my body is holding itself upright, which is a constant set of adjustments, I'm processing what I'm going to say next, I'm processing and understanding what the other person is saying – that's a huge amount of work. Any one thing that you do involves a phenomenal amount of processing, all the way from incredibly basic up to thoroughly complex and cognitive.
So yes, it's all working all the time. I believe the “10%” idea came from a self-improvement manual from about 100 years ago - the implication being that you could do so much more. What it's got turned into is “well, we've got all these brain cells but we don't even use them!” I once went on a first aid course, and someone in all seriousness said “brain injuries are not even a problem because we only use 10% of our brain”. Totally ludicrous, but that's how far this idea has become received wisdom.
“Because of the complexity, plasticity and flexibility of the human brain, it seems to have no limit at all in terms of how much extra information it can store.”
But, if you think about it in the sense that they originally intended it to mean - that you could do more - then actually there's a germ of truth there. Because of the boundless complexity, plasticity and flexibility of the human brain. It seems to have in practical terms, no limit at all in terms of how much extra information it can store.
The fact that humans can adapt to almost any climate on earth is the proof of this, this is because of the adaptability of our brains. We are an incredible predator, despite not having claws or fangs or not being particular strong by ape standards. Our brains allow us to make tools, to work in groups, to build cars and cities, yet - and this is a quite incredible thing - as far as we know, there is no difference between our brains and the brains of the first modern humans, hundreds of thousands of years ago.
Those first homo sapiens, given a modern upbringing, would have been capable of just what we're capable of. That is testament to how flexible the human brain is, and we've got no reason at all to believe that we have reached or approached any kind of limit. So while no, there isn't an inactive portion of your brain, the intention behind the idea – which is that you can do so, so much more with your brain than you think you can – is actually true.