What Are The White Blotchy Patches Spread All Over The Back?

I have white patches of skin on my back and normally it is quite tanned. It's blotchy and dotty, however doesn't itch or feel any different to the rest of the skin on my back. It started off small, but has now spread and is nearly all over my back. It's not scaly or raised, it is completely normal apart from the de-colourisation. What could it be?