The genre in short
IDM stands for intelligent dance music, a label almost none of the artists involved ever accepted. It comes from an American mailing list started in 1993, named after Artificial Intelligence, the compilation Warp had released the year before with the words electronic listening music on the sleeve and an armchair in the artwork.
The adjective is the problem: intelligent implies the rest is not. Strip the name away and what is left is music built for close listening, with rhythms that keep shifting and a level of detail that reads better on headphones than in a room. Aphex Twin, Autechre, Boards of Canada: the point was never difficulty, it was density.