What observations suggest the mass of the Galaxy goes much farther out than its visible disc?

Question

Quizplus · Accepted Answer

The rotation curve of the outermost portions of the disc suggests that the mass of the galaxy goes much farther out than its visible disc, indicating the presence of dark matter. The observed rotation of objects in the outer regions of the galaxy is faster than expected based on the visible mass alone. This can only be explained by the presence of additional, unseen matter, which implies the mass of the galaxy extends beyond its visible disc.

What Observations Suggest the Mass of the Galaxy Goes Much