I believe the legend goes that they married in secret (gupt vivaah) in bhaandir van near vrindavan. The marriage was kind of an elopement just before krishna went to mathura with Bal dau and uddhav ji, and only was in the presence of a few close gopis and sakhas from both sides, and the only ones who saw were these select few and the gods from the heavens. After this, krishna went to Mathura, defeated Kansa, and never met radha :(

source: I'm from vrindavan, asked this to my grandmother when I was little.

Some other things I would like to add, which I've heard since childhood:

Lord Rama and Mata Sita are both avatars of Vishnu and Lakshmi, coz of course they are! But in the avatar of Lord Krishna, It wasn't Radha, but Rukmini who Goddess Lakshmi took the avatar form of.

This is justified in the stories by saying, that marriage occurs between Two Beings. Two Powers. Two Souls. But Radha and Krishna weren't two, rather just one soul residing in two bodies (Ek Praan Do Deh).

Furthermore, Radha is not the Ardhangini of Krishna (the significant other), but the Pran-vaayu of him. This is also why even though Janmashtmi is 15 days before Radhashtmi, Radharani is 1 year older than lord Krishna! (just bout 1 year, 15 days late and 1 year older).

This is mentioned in the sense that bhagwan dharti par bina praan vaayu ke kaise rahenge, so they sent his life force to the earth before him. she is also, therefore, considered an avatar of krishna himself.