Working with a flat and infinite model of the universe (which seems to be the most popular serious model these days), the density of the universe is 3e-28 kg/m^3 . When added to the cosmological principle, it would imply that the mass of the universe is infinite.
Yet, the same umass.edu link  reports "...that the number of atoms in the Universe is at least about 4e78, but perhaps as many as 6e79." What am I missing here? Note that, before this statement, no-where am I referring to the observable universe. If you know where I have erred, it would be great if you could restrict it to the flat and infinite assumption and also talk in the context of the entire universe (and entirely skip references to the observable universe).