be in the mainstream (of something)

To be widely accepted or normalized. Lots of ideas that are currently in the mainstream weren't at one time. I mean, not that long ago, women and minorities in this country weren't allowed to vote. That hairstyle definitely isn't in the mainstream of fashion, yikes. The festival celebrates the weird and wonderful car designs that aren't in the mainstream.

get in(to) the mainstream (of something)

To become widely accepted or normalized. Lots of ideas that have gotten into the mainstream weren't accepted at one time. I mean, not that long ago, women and minorities in this country weren't allowed to vote. I hope that hairstyle never gets into the mainstream of fashion—yikes! The festival celebrates the weird and wonderful car designs that haven't gotten into the mainstream.

in the mainstream (of something)

Widely accepted or normalized. Lots of ideas currently in the mainstream weren't at one time. I mean, not that long ago, women and minorities in this country weren't allowed to vote. That hairstyle definitely isn't in the mainstream of fashion, yikes. The festival celebrates the weird and wonderful car designs that aren't in the mainstream.

mainstream media

Well-established media outlets, such as newspapers and TV stations, that reach a mass audience, considered collectively. The term is especially used to contrast it with other forms of media that one considers more truthful and/or less biased. Often abbreviated as "MSM." It seems that people are becoming increasingly distrustful of the mainstream media.

*in the mainstream (of something)

following the current trends or styles that are popular or are considered normal. (*Typically: be ~; get [into] ~.) Bob is too old-fashioned to be in the mainstream of modern living.