Abstract: Astronomy has always been a science of catalogs: catalogs of stars, galaxies, quasars, and planets. While most of the light in the Universe comes from these dense objects in the darkness, the contents of the universe are largely diffuse. Dark energy, dark matter, plasma, and gas make up 99.8% of the mass-energy budget of the Universe and cannot be easily cataloged. If we want to understand how the objects in the universe came to be, we must appeal to the largely invisible diffuse phase that formed them. I will try to make sense of this conundrum in three ways. The first is to use the cataloged objects to discern things about the intervening gas: through dust reddening we can measure the location and density of metal enriched gas in the Galaxy and beyond. The second is to examine the velocity-resolved emission from the gas. To do this we invent statistically robust ways to measure the shape structure of the gas, where much of the information about the diffuse phases hides. The third is to combine the two, catalogs and continuum, to delve into the 4D structure of our Galaxy. I will discuss a method in development, Kinetic Tomography, which will hopefully help us understand how the Galaxy came to be the way it is today.
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