One spot in the universe is colder than the Big Bang's leftover heat.

Empty space is not at zero. The whole cosmos is bathed in the cosmic microwave background, the faint afterglow of the Big Bang, still glowing at about 2.7 Kelvin: a few degrees above absolute zero. Every rock, every cloud of gas, every patch of vacuum sits in that bath. It is the temperature floor of the universe. Nothing natural should be able to get below it.

Diagram: 0 K · Cosmic floor: 2.7 K · Rock · Gas cloud · Vacuum · Boomerang Nebula: 1 K · the only natural place below the floor · Empty space rests at 2.7 K, the Big Bang's afterglow. One dying star fell below it.

5,000 light-years away, the Boomerang Nebula reads about 1 Kelvin: colder than the 2.7 K background that surrounds it. It is the only known natural place in the universe below that floor. The cold has a mundane source. A dying star is flinging gas outward at over 150 kilometers per second, and as that gas expands it does work and loses energy, so it cools. The same physics chills the air rushing out of an aerosol can, or the coolant in your refrigerator. The nebula is expanding so fast it has refrigerated itself below the background heat of creation.

The coldest place we know of is a dying star, quietly running a fridge.