In the standard model of cosmology, the mass ratio of baryons to dark matter is about 1 : 5. The mass of the outer disks in nearby (z ~0) massive galaxies are usually dominated by dark matter, which explains the observations of the common flat or slightly rising shapes of galaxies' rotation curves in the local universe. However, a recent study reported that the rotation curves of star-forming galaxies at z ~ 0.6-2.6 may commonly be falling with radius. In this talk, I will discuss this study in detail about their observations, sample selection and methodology. Finally we will look at some implications from the results obtained.