If entropy is a real thing, a real quantity like mass and energy, then we should have some idea how much there is of it in the average cubic kilometer of space The standard metric unit of entropy is joules per kelvin. To get a quantitative grip on the amount of entropy in the universe lets estimate the amount contributed by all matter and radiation that has been detected so far. Arcane stuff like dark matter and undetectable unmeasurable but assumed low-energy neutrinos and dark energy which we dont know what it is we will leave out of the picture. So think of all the matter and radiation in sight, think about its entropy. How much joules per kelvin per cubic kilometer is that? Which of these is it closest to? 6 joules per kelvin 6 millijoules per kelvin 6 microjoules per kelvin 6 nanojoules per kelvin It turns out the vast majority of the entropy is in the Cosmic Microwave Background so that a first order estimate is just the CMB entropy per cubic kilometer. One of these is in fact that.