What is this bag of veins behind my left testicle and is it abnormal?

Near the testicle...: Is the epididymis, which is composed of numerous tubes, and is palpable in the scrotum. The epididymis may harbor cysts, making it more prominent. A large one is called a spermatocele. A varicocele is a collection of dilated veins that can be felt near the spermatic cord. Usually these entities are left alone unless large or painful. See your urologist for evaluation.