I am mid 30s and told from ct scan that i have a "low density" 6mm lesion on my kidney that they believe is a cyst but too small to characterize. ultrasound after saw nothing. really worried. are kidney lesions common? is low density a good thing?

Cysts in the kidneys are very common. I recall reading at one time cysts were present in roughly half of people over age 50. Cysts in younger people occur often as well. Personally, I would not worry if I was told I had a small renal (kidney) cyst. I understand that due to the small size, they can't say with 100% certainty it's a cyst. Statistically, a cyst is most likely, I would not worry.