An estimate of the amount of dark matter in the inner Milky Way is reported online in Nature Physics this week. Calculating the contribution of dark matter to the overall mass of the Milky Way and the Solar System has been challenging because of the indirect nature of dark matter measurements.

To estimate the mass of the dark matter, Fabio Iocco and colleagues first compiled up-to-date measurements of the ‘rotation curve’ of the Milky Way, giving the speeds of stars at fixed distances from the Galactic Centre, with the orbital speed related to mass through the law of gravitation. They then compared these measurements with the predicted speeds based on new models of the distribution of baryonic (normal) matter, which accounts for the mass of visible stars and gas in the Galaxy. The difference between these two quantities was then used to calculate the contribution of dark matter to the mass of the Milky Way.

The results provide compelling evidence for the presence of dark matter within the inner Galaxy, including the Solar System, though further research is necessary to determine its distribution and nature. This latest estimate may help reveal the structure and evolution of our Galaxy as well as guide future searches for dark matter, the most abundant type of matter in the Universe.