Mainstream culture doesn't exist? That's hilariously wrong. Kanye West would like you to twerk to some Hunger Games and The Hobbit, and take a selfie on Snapchat while #hashtagging about it on Twitter. Just don't take a naked one like Jennifer Lawrence. And don't forget to catch The Big Bang Theory next Sunday! Isn't the Harlem Shake so last year? And Psy, oh my God, who listens to Gangnam Style anymore?

If you think mainstream culture doesn't exist, you're probably stuck right inside it -- or so far from it that you don't even realize it's there. Oh well, back to playing some CS:GO.

I've heard of most of those things (except "CS:GO"). I have directly experienced some of them. My parents

I don't think "famous" is the same as "mainstream". For one thing, anything captured in digital form can break out to "famous" from any niche, using Internet's multiplicative effect. Is Psy mainstream because one of his hundreds of songs got a billion views on YouTube? Or is "Gangnam Style" mainstream, while Psy is niche?

Agreed, and if we can also agree that there are likely varying amounts of cultural involvement and self-awareness in any broadly painted group of humans, I think this follows:

It seems the root cause and motivation of many people who get classified as "hipster," who aren't simply reacting to or against whatever is the prevalent culture around them, comes directly from this Internet-driven, information-saturated, always-on media environment. It also directly results from how identity politics and marketing are dovetailing to constrain peoples' ideas of self-agency. People performing whatever actions are considered "hipster" are doing so from a position of ambivalence toward being constrained in such a way. They are resisting categorization, specifically because 1) it seems to immediately lead to someone trying to sell you something based on limited or superficial evidence, and 2) because, like the above commenter mentions, there is no longer a "mainstream". It's dissolved into countless iterations of pastiche. The people who have seen this all happening have determined that they'd rather select from the detritus of cultures past, as it's all being recycled anyway, and not subscribe to ideologies beyond what interests them in the moment.

All this also comes off as quite superficial and easily derided. But beneath the derision, I think the loudest anti-hipster voices are upset that this kind of person doesn't seem to attach themselves to an Ideology. This must terrify some people.

There still is a mainstream, it's the old vanguard. Things like the Billboard list artists, TV networks and pop culture celebrities (think Lady Gaga, Katy Perry, Jay Z, Beyonce) are all part of what is considered mainstream. It may not be as culturally dominant as it was in the past but it does still exist.