All canonical information regarding Dark samus can be found here: 
Dr. Anonymous' Version Prior to the events of Metroid Prime 3: Corruption, the bounty hunter Sylux happened upon her home planet of Phaaze. Drawn by the amount of energy his ship's scans had picked up on the planet's surface, he flew down toward Phaaze to check it out. Upon landing, his ship were captured by Dark Samus, and an unfortunate Sylux was converted into a basic template which Dark Samus could use to construct a new body in case her current one were to be destroyed.