I was reading an Asimov book the other day and came across something called Skewes' number. At one time, Skewes' number was the largest number ever to appear in a mathematical proof.1 It would be impossible to write Skewes' number out in the conventional way, but using exponents we can write it as
This is an exceedingly large number. It is vastly bigger than the number of grains of sand in the Earth. In fact, it's vastly bigger than the number of grains of sand in the solar system. In fact, it's vastly bigger than the number of atoms in the solar system. In fact, all of these physical quantities are extreme underestimates of Skewes' number. What physical quantity comes closest to Skewes' number?
Note: It must be something physical like a molecule, atom, or grain of sand (i.e. not something abstract like a number appearing in a mathematical proof.)
 It has since been replaced by other large numbers. For more info, see Graham's number and Moser's number.