The leading theory, so far, is that it's an icy material such as water ice, but some kind of mineral deposit is also a possibility. Now, with the help of the HARPS spectrograph attached to the ESO 3.6-meter telescope at La Silla, Chile, it seems the ice theory has just become a whole lot stronger.
ANALYSIS: Mystery Mountain Pops Up in Striking Ceres Photo
"As soon as the Dawn spacecraft revealed the mysterious bright spots on the surface of Ceres, I immediately thought of the possible measurable effects from Earth," said Paolo Molaro, at the INAF–Trieste Astronomical Observatory and lead author of a study published in the journal Monthly Notices of the Royal Astronomical Society. "As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth."
As Ceres rotates every 9 hours, HARPS is so sensitive that it can detect the very slight Doppler shift in spectrum frequency as the bright spots rotate toward and away Earth, but during observations for 2 nights in July and August 2015, more changes not related to Ceres' spin were detected.