Throughout the entire first season of the new X-Men show The Gifted, they talked about the event that made the X-Men and the Brotherhood vanish. It's known as "7/15", and we don't know that much about it. We've seen it take place, but from far away, and it looks like some sort of fight or terrorist attack. The mutants got the blame, and things went to hell from there. It's a big event and it was only really touched on in season one. Showrunner Matt Nix recently spoke to TV Line and said that the event is something that they would like to explore in season two.
"I would definitely like to explore 7/15 in more detail" and possibly via flashbacks, showrunner Matt Nix said. "What exactly happened there is certainly one of the central questions of the series," and thus wouldn't be recounted within just a singular episode. "It's a bigger deal than that," Nix notes, "linked as it is to the disappearance of the X-Men and the Brotherhood, the creation of the Mutant Underground and the attempted rebuilding of the Hellfire Club…. It's kind of at the center of everything we're doing."
It's a big event that triggered a major reaction in this version of the universe. Not knowing about it isn't really a bad thing, but it could help explain the motivations of not only the humans, but the mutants has well. Now that we have the mutants splintered between the Underground and a new version of the Hellfire club, explaining each of their motivation is going to be important in moving the show forward.
The Gifted, created by Matt Nix, stars Stephen Moyer, Amy Acker, Natalie Alyn Lind, Percey Hynes White, Sean Teale, Emma Dumont, Jamie Chung, and Blair Redford. It is expected to return in late 2018, but that is not definitive.