What Causes A Growth On The Areola?

Hello,

I am a 61 yr. old white male. I have a growth on the areola of my left breast below the nipple.It is about 2cm x 2 cm and extends from the edge of the areola to about 1 cm below the nipple. The color is darker than the surrounding areola tissue,and the surface is rough/thick.

It has been there for at least a year that I have noticed it.What might it be?

Thanks,

Dan