She’s had a few scenes that mention her training and life as an assassin, and we got to see a younger version of Natasha during the character’s vision in Avengers: Age of Ultron.
The Black Widow movie has been a long time coming, and has the potential to finally expand the character’s mysterious backstory.
Still, she’s an OG Avenger who has largely functioned as a supporting character in The Avengers, Captain America: Winter Soldier, and Iron Man 2.
While comic book fans may know everything about Natasha Romanoff’s past on the page, it’s not something that’s been shown in great detail in the MCU.