Balance of Dark and Luminous Mass in Rotating Galaxies

A fine balance between dark and baryonic mass is observed in spiral galaxies. As the contribution of the baryons to the total rotation velocity increases, the contribution of the dark matter decreases by a compensating amount. This poses a fine-tuning problem for $\Lambda \mathrm{CDM}$ galaxy formation models, and may point to new physics for dark matter particles or even a modification of gravity.