Even though people always think of the brain as being "grey matter", there is actually more white matter in there than there is grey matter.
As it says in the article, the white matter is mostly made up of myelinated axons. These are like thin fibers.
In fact, I read recently that there is around 175,000km of myelinated axons in the brain of a young man. There is more like 150,000km in the brain of a young woman.
I'm not sure why there is that kind of difference, but it does add to the whole "Men are from Mars, Women are from Venus" debate, doesn't it?