Hello. are simple ovarian cysts usually cancer, i have a 5 cm simple cyst. its filled with fluid?

No, that would be exceedingly unusual. By definition, a cyst is filled with fluid and a simple cyst has no features that make it suspicious. Cysts come and go normally with your cycles and typically represent eggs at some stage towards ovulation. If a simple cyst persists over several cycles, continues to grow, or becomes symptomatic , it might need further management but cancer is still unlikely.