There has to be some sort of mass within space...and by that size of the Death Star, it must held a lot of mass even within space. It wasn't theoritically impossible to move something in that size.
Another question, why was the Death Star so defenseless? I mean, they just possibly just sented out only about 20-40 TIE Fighters and they still lost? Why weren't there any backup fleets or escorts? Were the Imperials so cocky?! Damn Admiral Moff Tarkin!