I stumbled upon a YouTube video of the game.
Months ago, when Last.FM wasn't terribly broken and I was still using it, I loved to check pages of Japanese artists, which music I was listening. On last.FM, there was often a link to youtube video on the song page, so you could listen to it. There was also a nice feature to find "similar artists", from which I was making a good use.
However, you know how it is with Japanese music randomly uploaded on youtube and copyrights - it often disappears. Thus, some people found a solution - linking youtube videos with osu! gameplay replays of the songs, if they were missing.
At first I was mostly ignoring them, as I was only listening to music and hitsound were annoying. Nevertheless, while listening to anime music, both at last.FM and directly on youtube page, I was encountering osu! replays quite often. I checked some of them and started to wonder "what game it is?"
I quickly got interested by them. I don't remember what song it was, but after one of these videos the dam get broken and I told myself: "Enough! I am checking this game, I must to try it!"