Dark Star is a holosseum stage in Custom Robo (GameCube) and Custom Robo Arena. It is set in outer space, but as the battle progresses, the field turns dark and becomes invisible. This makes it very diffcult to find obstacles to hide behind. Also, be careful of distance because it is hard to gauge on this stage.
Custom Robo (N64)
Dark Star is the 2nd stage that you fight Shinonome on. It becomes available after you beat him on it.