Grimes confirms birth of second child with Elon Musk

March 10 (UPI) -- Canadian singer Grimes is a mom of two.

The 33-year-old recording artist and her partner, Tesla and Space X founder Elon Musk, secretly welcomed their second child, Exa Dark Sideræl Musk, via surrogate.

Grimes shared the news in the April issue of Vanity Fair. She said Exa is a reference to the supercomputing term exaFLOPS, while Dark references "the unknown."

"People fear it but truly it's the absence of photons," Grimes said. "Dark matter is the beautiful mystery of our universe."

Sideræl, meanwhile, is "a more elven spelling" of "sidereal," or "the true time of the universe, star time, deep space time, not our relative earth time," and a nod to the Lord of the Rings character Galadriel.

Grimes and Musk call their daughter "Y." The couple also have a son, X Æ A-Xii, or X, born in May 2020.

Grimes and Musk made their red carpet debut as a couple at the Met Gala in May 2018. Musk told Page Six in September that he and Grimes were "semi-separated" but on good terms.

In the new interview, Grimes said she and Musk live separately but are still involved and plan to have more children.

"There's no real word for it," she said of their relationship status. "I would probably refer to him as my boyfriend, but we're very fluid. We live in separate houses. We're best friends. We see each other all the time... We just have our own thing going on, and I don't expect other people to understand it."

Grimes released her fifth album, Miss Anthropocene, in February 2020, and has since released the singles "Player of Games" and "Shinigami Eyes." She plans to release a new album, Book 1, in 2022.