Using much of the same reasoning by which the Planck time and the Planck length are derived, it is also possible to calculate the mass and density of the universe at the Planck time. It turns out that the Planck mass minus the mass of the universe 10–43 second after the Big Bang is about a thousandth of a milligram.

That does not sound like much by terrestrial standards. Remember, though, that mass is contained within a volume that is less than a hundredth of a billionth of a billionth the diameter of an atomic nucleus. So the density of that primordial universe is an incredible 10⁹⁴ times the density of water. Nothing in our universe today that we know of, including the densest known black holes, even remotely approaches that kind of density. Energy this concentrated must certainly behave in ways almost unimaginable in the current universe, and that is almost certainly reflected in everything that happened in the infant cosmos.