Why is the Boomerang Nebula the coldest place in the universe?
As noted in your link, this nebula is the result of rapid expansion of gas dust on a very large scale. You can see similar effects on earth when compressed gas is released from an enclosed space. The most energetic molecules rush outward faster and the ones with very little kinetic energy are left behind. Since average kinetic energy in a collection of gas molecules is what we measure as temperature, we would say that the slow ones are colder. Note also that this is just the coldest spot we have measured, and it is relatively near the Earth (5000 light-years). There could be colder spots buried in far distant galaxies. However, at 1 degree Kelvin, this one is pretty close to the limit of absolute zero, where there is no measurable kinetic energy left.
