On December 4, 2015, Starship Entertainment and Yuehua Entertainment launched a 12 member girl group called Cosmic Girls, where the Korean and Chinese members were divided into different subunits.
On December 10, the first unit, Wonder Unit, was announced (Cheng Xiao, Bona and Dayoung). On December 17, the second three-member unit (Xuan Yi, Eunseo and Yeoreum) was introduced: Joy Unit. On December 21, Cosmic Girls' Wonder and Joy Units released a Christmas cover of "All I Want For Christmas Is You" by Mariah Carey. On December 24, the third unit, Sweet Unit, was announced (SeolA, Exy and Soobin). On December 31, the final unit was announced: Natural Unit (Mei Qi, Luda and Dawon).
Cosmic Girls photographed during the closing ceremony of Yonsei University on May 26, 2016.
Cosmic Girls debuted on February 25, 2016 with the release of their debut mini album Would You Like?, including the title tracks "Mo Mo Mo" and "Catch Me". The group held their debut stage on the February 25th episode of M Countdown with "Catch Me" and "Mo Mo Mo".
On July 11, Starship Entertainment confirmed that I.O.I's Yoo Yeon-jung would be joining the group. In August 2016, SeolA, Exy, Soobin, Eunseo, Cheng Xiao, Yeoreum, and Dayoung teamed up with label mate Monsta X to form the unit "Y-Teen." Y-teen was a project unit group that promoted as CF models for KT’s phone fare service and would release EPs, music videos, and various entertainment content.
On August 17, Cosmic Girls released their second mini album The Secret with all 13 members.
2017: From. WJSN, first concert and Happy Moment
Cosmic Girl held their first solo concert, Would You Like - Happy Moment, from May 19–20 at the Blue Square Samsung Card Hall.
Cosmic Girls in 2017
Cosmic Girls' first full-length album, Happy Moment, was released on June 7, 2017, with a total of ten tracks including the lead single "Happy". The album was topped sales by taking the number 1 spot on the Hanteo's daily and real-time charts.
On February 27, 2018, Cosmic Girls released their fourth mini album Dream Your Dream, which consists of six tracks including the lead single "Dreams Come True". Their comeback showcase was held at the Yes24 Live Hall on the same day as the album's release.
On June 1, 2018, Cosmic Girls and Weki Meki formed a project group called WJMK and released a digital single "Strong".
Two Chinese members (Mei Qi and Xuan Yi) participated in the Chinese version of survival show Produce 101 during the first half of 2018. The survival show concluded with Mei Qi ranking 1st and Xuan Yi ranking 2nd, making them both members of the temporary girl group Rocket Girls 101.
On September 19, 2018, Cosmic Girls returned with their fifth mini album WJ Please?. The Chinese members, Cheng Xiao, Mei Qi and Xuan Yi, did not participate in the comeback and continued as ten members. Mei Qi and Xuan Yi are preparing to debut with Rocket Girls 101, while Cheng Xiao is filming Legend of Awakening, a Chinese historical fantasy drama as the second lead actress, which would be her acting debut.
On October 2, 2018, the group received their first music show win on SBS MTV's The Show.
On January 8, 2019, Cosmic Girls released their sixth mini album WJ Stay?, which consists of seven tracks including the lead single "La La Love". The group continued to promote as ten members, with the Chinese members, Cheng Xiao, Mei Qi and Xuan Yi, unable to participate in the comeback due to prior commitments.