The real fact is that Jeffrey Dahmer was a serial killer. His victims were real people. The series shows his pattern of luring, killing, and dismembering his victims which were part of the real events.
Well, one of the real facts in the Dahmer series is the discovery of his crimes. In real life, the police eventually found evidence in his apartment that led to his arrest. Also, his modus operandi of targeting young men, often from marginalized communities, is a real aspect shown in the series. His strange and disturbing behavior towards his victims was also real, like his attempts to preserve body parts. All these are facts that the series is based on.
The fact that Dahmer had a troubled upbringing is real and shown in the series. His family life, with his parents' divorce and his own feelings of isolation, was part of what might have contributed to his actions. Also, the fact that he managed to evade the law for some time while continuing his crimes is real. The series also accurately portrays the public outcry and shock when his crimes were finally exposed, as this was a very real reaction in society at that time.