When the general public speaks about dark matter, most of the discussion centers on the topic of whether dark matter exists or not: on whether it’s real. But in astrophysical circles, we’re overwhelmingly confident of dark matter’s existence, as we’ve indirectly detected it through an enormous number of independent lines of evidence. Instead, the conversation normally focuses on how to identify and characterize it, with a particular interest in what sorts of measurements can help reveal its underlying nature.

We can learn a whole lot about dark matter’s nature simply by observing the luminous signatures of the normal matter within galaxies. Matter follows a path dictated by gravity, and so if you can find a signature within a galaxy that will lead to very different observables depending on the shape, profile, and clumpiness of a dark matter halo, that can help teach you which models of dark matter remain viable and which ones are disfavored by the observations.…