This is actually a common misconception about how jet engines work. The engine is not pushing against the surrounding medium (the air), it is operating purely due to the conservation of momentum of throwing combustion byproducts at a high velocity in the opposite direction. This is the same principle upon which a rocket engine operates, the difference being that the jet only needs to bring along fuel for the combustion reaction, thereby massively increasing the efficiency of the engine by making use of ambient atmospheric oxygen.
In fact, a jet engine could in theory (ignoring things like heat rejection) operate just fine in space if you brought along a massive bag of air and hooked it up to the intake duct. But then you've basically just designed the world's worst rocket engine :)
In fact, a jet engine could in theory (ignoring things like heat rejection) operate just fine in space if you brought along a massive bag of air and hooked it up to the intake duct. But then you've basically just designed the world's worst rocket engine :)