I'm pretty sure there isn't a universal cure for cancer. Some forms are more treatable than others and we have made a lot of progress. The conspiracy guys are crazy imo cause there are tons and tons of scientists who have looked into this, even ones not studying cancer specifically have enough knowledge to understand how difficult it would be to come up with something to cure it. Btw "cancer cells" represents a much larger category of abnormal cells, thats why skin cancer is different from prostate cancer.
Even with my meager understanding of cell biology I know that cancer can arise from nowhere, and spread really fast (HeLa cells). Any cure I can think of would either have to kill the cells or reverse their transformations, the latter being possible but completely impractical and the former being incredibly tricky.