It is true that there is not enough material in the universe to express Graham’s number by conventional means, so Donald Kuth devised special notation where 3^3 means three cubed and 3^^3 means 3^(3^3) i.e 3^27.
3^^^3 = 3^^(3^^3) which would, if expressed conventionally give a tower of exponents 7,625,597,484,987 layershigh!
Now consider the number 3^^^…^^^3, in which there are 3^^^^3 arrows – an unimaginably large number.
Then the number 3^^^…^^^3 in which the number of arrows is that previous number.
Carry on doing this, making the number of arrows in 3^^^…^^^3 equal to the previous number until you are 63 steps from 3^^^^3. That is Graham’s number.
(Thanks to the Penguin Dictionary of Curious and Interesting Numbers)