The science

The outer edges of spiral galaxies orbit far faster than the matter we can see can explain. In NGC 3198, a galaxy about 45 million light-years away, gas nearly 150,000 light-years from the center still orbits at about 150 km/s, while the stars and gas we can see account for only about 65 km/s there.

In the solar system, orbital speeds fall with distance: Neptune moves about 9 times slower than Mercury. Spiral galaxies don't behave that way. In the 1970s, radio astronomers (Roberts, Rogstad and Shostak, Bosma) and, with optical spectra, Vera Rubin and Kent Ford found that the outer parts of spiral galaxies orbit roughly as fast as the inner parts. By 1980, Rubin, Ford and Thonnard called unseen matter "inescapable".

The leading explanation is dark matter. Planck's measurements of the cosmic microwave background imply about 5 times more dark matter than ordinary matter, roughly 84% of all matter. What it is made of is unknown, and no experiment has confirmed a dark matter particle. A minority of researchers instead propose changing the law of gravity at very low accelerations (MOND).

The galaxy in the video is a model driven by NGC 3198's measured speeds, and the violet halo shows where the unseen mass would have to be; it has never been seen directly.