Why does the clustering of dark matter and normal matter happen together?
Dark and luminous matter don’t form structures entirely independently of each other. Although only luminous (ordinary) matter can have electromagnetic interactions (emitting and absorbing electromagnetic radiation), which affect how it forms into clumpy structures, both luminous and dark matter have mass, and therefore have gravitational interactions. Dark and luminous matter interact with each other via gravity; they orbit each other, and in fact dark matter plays a significant role in the formation of structure of ordinary matter. The reason we have a structure in which highly clumped luminous matter is surrounded by more diffuse dark matter is primarily because luminous matter clumps more easily due to its extra interactions: it can radiate away orbital energy as it collapses, and in addition can stick together more easily via electromagnetic forces. Therefore, you get denser clumps of ordinary matter; because any matter (dark or luminous) is more attracted to denser clumps, diffuse
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