When you're an expert at something, you show expertise — or an exceptional knowledge of the subject. "He had expertise in accounting, having excelled in math, but he had even more expertise in dancing, his hobby since a small child."
Expertise has obvious roots in the word expert. Just about any interest or field has a place for experts. Just as one individual has expertise in world religions, another may have expertise in computer gaming. Working hard or immersing yourself in a specialty leads to expertise, and it comes through both study and hands-on experience.