There are several potential causes of this dark spot between your breasts. I would suggest that you make a visit with either your dermatologist
or your primary care doctor
. They will be able to examine the spot and help figure out what it is.
If the spot has been present for a long time, it is possible that it is a birthmark or a mole. Most of the time these are not a major concern, unless they are new or changing in size or appearance, in which case your doctor may want to biopsy
them to rule out a skin cancer.
On the other hand, if the spot is relatively new, it could be a fungal infection. Other symptoms that would go along with a fungal infection on the skin would include itching, flaking or scaling, and potentially an odor. Fungal infections generally clear up nicely with topical anti fungal medications, which your doctor could prescribe for you.
Finally, I think this could be eczema or contact dermatitis, two inflammatory reactions in the skin. Unlike fungal infections, eczema and contact dermatitis are treated with topical steroid creams not topical anti fungals so it is important to distinguish between the two even though they sometimes look alike.
Start by seeing your doctor as soon as you can!