Jenn Stroud Rossmann, an assistant professor of engineering at Harvey Mudd College, provides this explanation:
Jets leave white trails, or contrails, in their wakes for the same reason you can sometimes see your breath. The hot, humid exhaust from jet engines mixes with the atmosphere, which at high altitude is of much lower vapor pressure and temperature than the exhaust gas. The water vapor contained in the jet exhaust condenses and may freeze, and this mixing process forms a cloud very similar to the one your hot breath makes on a cold day.

https://www.scientificamerican.com/article/why-do-jets-leave-a-white/
And yes! When I breathe in the cold air, there is conspiracy and chemicals!