My Eyebrow is Partially Bald? ?

I just noticed it today, but one eyebrow has very thing, or hardly any hair about half way through it, it's not quiet bald but pretty empty too, compared the other, which is full. I'm only 15, I haven't plucked at them or anything - I'm just wondering if it will fill in or not? It's not that I'm extremely vain, but would like my brows to have some symmetry D: