It's the oils/chemicals in the dash getting frozen. I haven't been in extremely cold weather yet, but in say 30-40* weather, it disappears on mine once it warms up and thaws out. The passenger side air bag in the dash also sinks in a little so you can see a noticeable impression. That too returns to normal after warming up.
I read somewhere (this or another forum) that a product called Turtlewax Ice Total Interior Care will help prevent this, as well as restore the dash in case it doesn't turn back on its own. I live in northern VA, my Challenger is outside 24/7, and we've had it as low as 14 degrees some mornings this past winter - and long cold spells where it never got above the low 30s. I used this product on my dash before winter set in and have not had this occurance. Not entirely sure its the product or just lucky on my part - I do like the product and would use it anyway - no glossy or greasy finish; makes the dash and other interior (non metal) parts look brand new. Will cover any minor scratches in the plastic too. But, I didn't have the white blotching/spotting that some have had in colder climates, so maybe it does help.
It's a chemical release agent that allows the dash to come out of the mold. It does appear under cold conditions. You simply need to clean your dash. I strongly recommend doing so BEFORE doing any other treatment as the other treatment can seal the chemicals in.
To clean mine, I used a simple vinegar/water solution and a rag. Followed up with the 303 aerospace product as I am a new believer in it.