“Space and time may have a structure as intricate as the fauna of a rich ecosystem, but on a scale far larger than the horizon of our observations.” -Martin Rees
On the largest scales, dark matter has been undoubtedly the most successful theory in modern cosmology for explaining a huge variety of observations. From the motions of galaxies in clusters to the separation of mass and light when they collide, from the correlations between galactic positions to the fluctuations in the CMB, from the bending of starlight to the formation of large-scale structure, it’s clear that the Universe needs dark matter.
But individual galaxies have always been the most difficult test for dark matter. In particular, there have been empirical correlations — or relationships between two different observables — that have never had an underlying explanation successfully presented. One of the most difficult has been the Tully-Fisher relation, which relates the luminosity to the rotational speed of spiral galaxies. But a new simulation, at long last, has finally cracked that nut by incorporating not only gravitation and dark matter, but the relationship between baryons and dark matter.
The way a galaxy forms stars over its history matters tremendously for what we get today, and by simulating it all together, it adds up to one stunning conclusion: success for dark matter in an entirely new way!