(The Hill) — Elon Musk and Grimes welcomed their second child in December, a girl named Exa Dark Sideræl Musk, or “Y” for short. The child, whose birth was revealed in a Vanity Fair profile of Grimes published on Thursday, joins older brother X Æ A-XII, who was born in 2020. 

The news broke by accident when the baby started crying in the couple’s home while Grimes was in the middle of speaking with Vanity Fair reporter Devin Gordon. Gordon wrote that he heard something “vaguely like a lone cry from an infant upstairs” after Grimes mentioned that X was with his dad. 

“I can tell by the frozen look on my host’s face that she heard it too,” Gordon wrote in the profile. “So I brace myself to ask the strangest question of my career: ‘Do you have another baby in your life, Grimes?’ Her body clenches and she looks away.” 

“I’m not at liberty to speak on these things,” Grimes responded. “Whatever is going on with family stuff, I just feel like kids need to stay out of it, and X is just out there. I mean, I think E is really seeing him as a protégé and bringing him to everything and stuff. … X is out there. His situation is like that. But, yeah, I don’t know.”

The singer eventually acknowledged the new baby, however, confirming that she was born in December through a surrogate.

“She’s a little colicky, too,” Grimes said, laughing. “I don’t know. I don’t know what I was thinking.” 

As for what the new baby’s name means, Gordon explained that, “Exa is a reference to the supercomputing term exaFLOPS (the ability to perform one quintillion floating-point operations per second).”  Grimes told the reporter that dark is “the unknown. People fear it but truly it’s the absence of photons. Dark matter is the beautiful mystery of our universe.” 

The singer said in the Vanity Fair story that she plans to keep her daughter’s life more private. “The best situation here is me training the girl and [Musk] training the boy.” 

The couple announced their split in September, with Musk saying at the time that they “still love each other” and are “on great terms.” Grimes now says their relationship is complicated, adding that she would refer to Musk as her boyfriend.

“There’s no real word for it,” the singer told Gordon. “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 also noted that the couple hopes to have more children. “We’ve always wanted at least three or four.”