Dark hard bump on back. it's painless. hasn't changed size. had it for about a year. no pus or anything when squeezed. what could it be?

See Dermatologist : For any lesion on one's skin the best best is to see a dermatologist. Many primary care physicians are able to diagnose skin conditions, and if not refer to a dermatologist. One's skin is the largest organ and subject to many, many conditions. Some benign and some deadly. Don't take chance and have it examined soon. Good luck.