From LQWiki

The geek community has embraced Starwars as much as Lord of the Rings and Star Trek.

In Starwars, there is a group of people called the Jedi. (Luke Skywalker trains to become a Jedi Knight). The Jedi use the force and lightsabres to battle against bad guys, especially the evil Sith lords who use the Dark Side of the force.

Summary over- the darkside is the bad side. Thus a darkside cracker/hacker is a bad hacker.

See: Black hat hacker cracker hacker