The Data matrix was the positronic essence of Data's being. It contained all his memories and personality.

In 2379, Data attempted to transfer his matrix to his brother B-4. Initially, the attempt appeared to have failed, with Geordi La Forge believing that B-4's brain was just not sophisticated enough to be able to handle the matrix. After Data was destroyed aboard the Scimitar, his matrix soon began to surface in B-4. (TNG movie & novelization: Star Trek Nemesis)

During the 2380s, an extensive study was made of B-4 at the Soong Foundation by a dedicated science team and Geordi La Forge. Eventually the group succeeded in unravelling the Data matrix, allowing for all of Data's memories, personality and experiences to override B-4, essentially resurrecting Data. Data helped the Soong Foundation upgrade the prototype positronic brain as well as recreating the emotion chip originally developed by Doctor Soong. Data returned to Starfleet duty and achieved the rank of captain by 2386. By the time Data was interviewed by Jake Sisko in 2390, Data refused to discuss his "return". (ST - Countdown comic: "Number Two"; ST website: The Path to 2409)