Mysteries RV Project:

Target #2c: The focus of this target is on the event or process that created the dark patch on the right side of Iapetus as shown in the image below, beginning with the initiation of the event or process itself.

Image Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Source url: https://apod.nasa.gov/apod/ap090809.html

Outside commentary on Iapetus: https://en.wikipedia.org/wiki/Iapetus_(moon)