The rocket engine isn’t “pushing” against anything. A rocket is a reaction engine. All the chemicals that burn in the engine have weight, even after they turn into fiery exhaust. “Burning” fuel does not eliminate its weight – it simply turns it from a solid or a liquid into a hot gas. As the hot gas exits the engine, the engine responds in a “to every action there is an equal and opposite reaction” way. So the hot, fiery, high-speed, heavy exhaust shoots out of the engine in one direction, and the “equal and opposite reaction” occurs in the rocket, which goes in the other direction. See How Rocket Engines Work for details.