Answer: This a benign condition called tubular (or cystic) dilatation of the rete testes.

Simply, it is just dilated mediastinal tubules of the testicle. Nothing to see (or do) here. They are commonly seen with spermatoceles, which kind of makes sense, and more common in older (>55yo) men. Always wise to exclude any associated mass and Doppler can be nice to help with that (should be normal).

Reference: http://radiopaedia.org/articles/tubular-ectasia-of-rete-testes