the observed velocities of stars and other objects in the galaxy do not match the predicted velocities based on the visible mass present in the galaxy. This discrepancy suggests the presence of additional mass that is not visible, which is commonly believed to be dark matter. Dark matter is thought to be a form of matter that does not interact with light or other forms of electromagnetic radiation, making it invisible to current telescopes and other observational methods. The rotation curves of spiral galaxies provide one of the strongest pieces of evidence for the existence of dark matter.
