3. When driving a car, the chemical energy in gasoline is converted to heat (thermal energy) in the engine. Most of this thermal energy is then converted to mechanical energy, which makes the car move. What happens to the rest of the thermal energy?

Explanation:

As we know that the maximum of thermal energy will be converted into mechanical energy.( The thermal energy will be given to us by engine.)

But the total thermal energy will not be converted into the mechanical energy,  there will be some lost of energy. The lost of mechanical energy will be in multiple forms. 2 of them are

1.Heat : The mechanical energy heats up the engine and then it will lost some energy.

2.Noise : Some energy also lost in the sound or the noise comes out from the engine. It will consume some energy.

Also, the engine never be 100% efficient some amount of energy lost in environment also.

3. dark matter

empty space could also be a simple vacuum