The corotation circle can be used to probe dark matter in a galaxy. In barred spiral galaxies, such as the Milky Way, the stars in the bar rotate faster than the stars in the spiral arms, as they are closer to the centre of the galaxy. Calculations have shown that sufficiently massive dark matter haloes slow the rotation, causing the corotation radius to be greater than 1.4 times the length of the bar.[3]
Most measurements have found that the corotation radius is always less than 1.4 times the bar length,[3] leading to the conclusion that dark matter does not significantly influence galactic rotation.
However, a 2017 study found that the arms of galaxies rotate more slowly than previously thought, implying that dark matter does sometimes influence the rotation of a galaxy even when the corotation radius is less than 1.4 times the bar length.[4][5]
