Sounds like a stupidly easy question to find out with a quick internet search, but it’s not.
I don’t want to know the average surface temperature, or the average ocean surface water temperature, or read another article about climate change.
But that’s all I found in the past hour.
I’d like to know the average temperature of all molecules that comprise earth, or a best guess scientific estimate.
You will have a very difficult time finding this. The average temperature of all molecules on earth is absurdly difficult to calculate, nearly impossible to gather data on, and not something that’s very useful for any practical calculations so no one has bothered to do it.
Black body radiation is probably more what you’re looking for, I would suggest starting there.
Black body radiation was my thought as well. It may not be the average including the inner layers, but it’s the average at the crust. About -1°F according to Wikipedia.
To add to this, is probably hard because the composition of the interior of the earth is a lot of guesswork. We can only directly observe how much heat is coming out of it.
https://en.m.wikipedia.org/wiki/Thermal_history_of_Earth