Hi Mike,
This is not something you can diagnose yourself, so please see a doctor. It's most likely just a cyst, but it could be testicular cancer. Testicular cancer is most common in men ages 15 - 35, so you are definitely in that range. I was diagnosed with testicular cancer 8 years ago (it's one of the most curable cancers out there) and I wish I had not waited to see a doctor because by the time I did, it had spread to my lymph nodes and to my lungs, which meant I had to have chemotherapy. Had I gone to a doctor before it had spread I likely would not have had to have chemo. The chemo was very intense, plus it cost over $100,000. I did not have insurance when I was diagnosed but was able to get insurance to pay for part of it. Don't wait. See a doctor.
Is i.st the Vas deferens / spermatic cord?? take a gander at a physiology chart ( a male chart would be best - for obvious reasons) - look at the structure of your gonads - I *think* what you have found i]may be a very normal part of your teste