I've only just started playing this past week or so, but Osu feels like it's a game that has always been there in the internet zeitgeist. I remember trying it like back around 2011 or 2012 I think, but not sticking with it since I didn't like playing with a mouse. And even though it wasn't until 2019 that I got back into rhythm games after not touching any after Guitar Hero 3, I still saw rhythm games everywhere online. I had always assumed that once the Guitar Hero and Rock Band craze died down that Osu was the most popular rhythm game out there, purely from the sheer amount of content I saw coming out from it.
Looking back though, I'm not sure if that's because Osu was actually that popular back then or if it's because the internet communities I followed overlapped heavily with the Osu community. Anime, NND memes, all that.