To heat up 1 kg of water with 1 degree, you need 4181.3 Joule of energy. This is called the water specific heat capacity. But at the same time, Boltzmann discovered, that the hotter you want to make some material, you need relatively just a tiny, tiny bit of energy more for every degree hotter. It is immensely small, this difference, so it only counts when you think about huge things such as the universe. The interesting thing is, this tiny difference goes for all materials, whether it is metal or gas or water, doesn’t matter. The constant is the same, and it is 1.38064 x 10^-23 Joule per degree Kelvin.
Now back to the water. And the universe. Why do we find these values for water specific heat capacity and the ratio between energy and temperature as Boltzmann found? Because of the charateristics of the part of the universe we live in. They tell us something about this system. If we would live in another environment, we would find other values for those constants. So, if we relate these two constants, we can calculate, our system contains 3,028 x 10^26 kg of water.
If our system would contain only half of this amount of water, you would find a bigger value for the Boltzmann constant. This bigger value would then express relatively much more energy to heat up something with one degree more hot. Therefore, this water can be seen as some sort of an isolating system. It keeps the heat in.
The earth and the atmosphere contain about 1.4 x 10^21 kg of water. If you subtract this amount from the 3.028 x 10^26 kg, we’re still left with 3.0279 x 10^26 kg of water. So we are obviously surrounded by huge amounts of water in space, having the function of isolating the space we live in.