I have recently had a dermoid removed. I started getting pain in my ovary because apparently it had started twisting. Did they say how large the cyst was? The reason they want to take it out is because it will not go away and the larger it gets the more likely the cyst will torse, or turn, cutting off the blood to the ovary. Once this occurs that is when you have problems.
I know you are worried hon, I was when I was diagnosed as well. I will tell you this, 98 percent of dermoids are non cancerous. If there is any slight chance, they will do a biopsy right then and there. My doctor told me once she got it out there was no doubt it was a dermoid, it even had hair in it...
If you have any questions at all, please talk to your doctor. I asked mine everything, the worse case scenario, etc. Since I am young, 35 yrs old I had less chance of mine being cancer. Since you are even younger than I am, then I would say it is a very good chance it is not, but I would speak to your doctor for more information.