The universe looks like it would look if dark matter really did exist, says Joshua Hillerup. If you think about what we know about the structure of galaxies, the picture is embedded in a bigger, puffier, dark matter halo. That's why galactic halos of dark matter are bigger and puffier than the condensed disc made of ordinary matter. In principle, could a living being like a cat or a physicist in some sort of well-designed container be put into superposition for an appreciable period of time? Or is there a hard limit to what or how long could something be in superposition? It depends on what you feel about the foundation of quantum mechanics.