I had a 27cm ovarian cyst filled with reddish brown fluid. What causes a cyst fill with fluid opposed to tissue? What's the difference in symptoms?

Cyst. A cyst by definition is a fluid filled sac. In the ovary you can have solid tumors (benign and cancerous), mixed solid and cystic and just cystic (fluid filled). Each growth is a tissue type, which is the key to what type you get. You probably had a serous (fluid type) cystadenoma or an endometrioma (bloody fluid filled cyst)