This question is inspired from this question earlier today.
In Back to the Future, we see the DeLorean famously exiting the van (see below), but there's a lot of smoke, or at least a whitish gas of some form exiting the van. Why was this gas there in the first place (in-universe)?
I reason that if it was just for effect (in-universe that is), why didn't Doc get Marty to tape the DeLorean exiting the van?
It was likely steam from the DeLorean's nuclear power core. You can actually see some steam being emitted from these vents in the clip (at 0:47 onwards).
These "Steam Vents" were apparently part of the control system used to keep the time machine's nuclear reactor from overheating and would therefore have already been engaged prior to a new set of fuel rods being inserted (assuming, of course that the Doc has tested that the reactor actually works).
Out of universe, we learn from the same article that the gas was in fact CO2
Since the car now sported a bank of flared exhaust vents behind the nuclear reactor (which lies a Dodge Polaris hubcap for a base). Pike's staff rerouted the vehicle's original exhaust system to vent in front of the rear wheels. Two CO2 extinguishers were installed in the passenger compartment and the gas was piped to the exhaust bank, where a trumpet with a special built-in baffle expelled the gas, simulating a nuclear reactor's exhaust.
According to historical records, on October 26 1985, the minimum temperature was 48.9F (about 9.4C). So, we could assume that at 1AM in the morning on the day in question, the temperature was around 50F. That's reasonably cold.
You will note that the ground on the car-park seems to be wet; it is totally possible that the DeLorean had been rained on and so was wet. We could therefore conclude then that when the DeLorean's ignition was turned on, and if it was left for a while, the internal temperature of the van would have been greater than the external temperature. Pair this with the water being inside and you have water molecules heated to above the external temperature.
Consequently, when the door of the van was opened, the water was exposed to a cooler temperature outside. So, I would say that the opaque gas was simply water vapor that had accumulated inside the van (on the DeLorean) and was simply reacting to change in temperatures.
