Space is supposed to be a vacuum. No air, no water - no density at all. Nothing to push off. Space travel is a fiction.
propulsion in spaces works for the same reason that it works on earth, as I said earlier, it obeys the laws of motion. every action has an equal and opposite reaction.
I'm guessing that you think a jet engine works by pushing against the air? IT doesn't. A jet engine needs air to work, but only because the air is an element of the combustion process, it is the force of the gasses leaving the engine that cause the engine (and anything attached to it) to move in the opposite direction.
a Rocket engine carries its own fuel, and can operate in vacuum conditions, the force generated by the combustion process still moves the engine in the opposite direction to the exhaust gasses.
the vacuum in space does mean that there is no resistance to movement, so once an object is moving it will continue on its path at a constant velocity until another force acts upon it. that force might be an artificial force, for instance firing a rocket for a short time, or it might encounter the gravitational force of a large body.