Grimes explained her daughter’s name in an interview with Vanity Fair. Exa refers to the computing term exaFLOPS. Dark means “the unknown. People fear it, but truly, it’s the absence of photons. Dark matter is the beautiful mystery of our universe.” Grimes added that “Sideræl” means “the true time of the universe, star time, deep space time, not our relative earth time.”

Source: https://www.buzzfeed.com/ryanschocket2/grimes-change-exa-dark-siderael-name