many sectors

At the brown-bag today, Iggy Sawicki talked about one of my favorite ideas: Make the dark matter out of copies of the light sector, but many, many very sparsely populated copies. You can think of these as other "folds" of a brane on which we live, or you can think of them as having some quantum number different from everything in our sector. But the point is, you can make the dark matter without postulating any novel particles or interactions at all. The key is that you need to make many other sectors and populate them all sparsely, otherwise the dark matter doesn't appear collisionless (or close thereto).