Grimes has revealed that she and Tesla CEO Elon Musk had a second child via surrogate that has been kept secret.
The pair are already parents to son X AE A-12, known as X, who was born in May 2020.
During an interview at her home in Austin, Grimes said she had left the baby being looked after upstairs in the hopes that she could keep her identity a secret, but as soon as the infant began to cry loudly, she confirmed that she had a daughter in December 2021 with the SpaceX founder.
The eccentric artist revealed that the baby girl is named Exa Dark Siderael Musk, Y for short, whilst being interviewed in her home for the April edition of Vanity Fair.
Exa refers to the super computing term exaflops, which is the ability to perform one quintillion floating-point operations per second.
Dark is a reference to dark matter, which is a hypothetical form of matter that makes up a huge chunk of the universe, but that science does not yet fully understand.
And finally, Siderael, pronounced “sigh-deer-ee-el”, was described by Grimes as “a more elven” spelling of Sidereal, which is “the true time of the universe, star time, deep space time, not our relative earth time”.
The baby is also nicknamed Sailor Mars by Grimes and Musk, as a nod to the Sailor Moon manga series.
The couple were reported to have split in 2021, after Musk told Time magazine that they were “semi-separated”.
However, Grimes appeared to confirm they have reunited, telling Vanity Fair: “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 said that they are happy, but they “just need to be free”.
She also told the publication that she and Musk sometimes discussed whether she was a simulation created to be his companion, and that she often believed she was.
She said: “The degree to which I feel engineered to have been this, like, perfect companion is crazy.”
Musk, who was named Time 2021 Person of the Year now has seven children.
He had twin sons named Griffin and Xavier in 2004, and triplets Kai, Saxon and Damian in 2006, with his ex-wife Justine Wilson.
The pair also had another child in 2002, who died at 10 weeks old.
Grimes also revealed that she has already introduced her infant son X to digital audio software and taken him to a rave, which he left at 11.30pm.
She said that it would be best if she “trained” her daughter and Musk “trained” their son, as he is already being taught about Musk’s various business ventures despite being only two.
The couple have not been strangers to controversy and drama, after Grimes publicly disagreed with Musk on Twitter in July 2020.
The billionaire tweeted “Pronouns suck”, to which Grimes responded saying she could not “support hate”.
Her response, now deleted, read: “I love you but please turn off ur phone or give me a dall [sic].
“I cannot support hate. Please stop this. I know this isn’t your heart.”