An opinion can start to look like a fact whenever an expert or trusted authority figure states it. A medical doctor on television can say that diet sodas are more dangerous than regular sodas, for example. That may sound like a fact, because we assume a doctor has done some research on his own or has access to scientific studies. However, the key word is "dangerous". There are many ways to interpret that word, and not all of them are based on scientific fact.
A diet soda bottle could fall on someone's foot and break a toe. In that sense, all bottled sodas could be considered dangerous. The ingredients in diet soda could trigger allergic reactions in test animals. That would also be considered dangerous, but in a different way. The doctor stated an opinion based on a set of facts, like the results of a study, but it is still only an opinion.