The number googolplex is so big there is not enough space in the universe to write it down.

A googolplex is a ten powered by a googol of zeros. It is so big, that there is not enough space in the entire observable universe just to write it down, even if you could write a single zero on each atom.

In the PBS science program Cosmos: A Personal Voyage, Episode 9: “The Lives of the Stars”, astronomer and television personality Carl Sagan estimated that writing a googolplex in standard form (i.e., “10,000,000,000…”) would be physically impossible, since doing so would require more space than the known universe provides.

An average book of 60 cubic inches can be printed with 5×105 zeroes (5 characters per word, 10 words per line, 25 lines per page, 400 pages), or 8.3×103zeros per cubic inch. The observable (i.e. past light cone) universe contains 6×1083 cubic inches (4/3 × π × (14×109 light years in inches)3). This math implies that if the universe is stuffed with paper printed with 0s, it could contain only 5.3×1087 zeros—far short of a googol of zeros. In fact there are only about 2.5×1089 elementary particles in the observable universe, so even if one were to use an elementary particle to represent each digit, one would run out of particles well before reaching a googol digits.

Consider printing the digits of a googolplex in unreadable, one-point font (0.353 mm per digit). It would take about 3.5×1096 metres to write a googolplex in one-point font. The observable universe is estimated to be 8.80×1026 metres, or 93 billion light-years, in diameter, so the distance required to write the necessary zeroes is 4.0×1069 times as long as the estimated universe.