That unallocated space is the over partitioning section that is configured by the Samsung Magician software if you select it when you set it up. When an SSD becomes full the performance goes down badly so to prevent that they advise that you over partition so that when you run out of partitioned space the drive will still have room and maintain performance. So if this is not a concern for you and you feel you'll never use up all of the room you simply need to right click and choose extend the volume.