The world – well Gagosian Gallery, anyway – has gone dotty for Damien Hirst’s spot paintings. In an unprecedented move, Gagosian is showing a single artist across all its sites, and not just a single artist but a single strand of that artist’s work. The Complete Spot Pantings 1986-2011 is on now at all 11 Gagosian spaces globally. Why? What’s it all about?