In reality there is no answer to the question: What is the highest number known to man? With the addition of 1, any number presented as the largest can be made obsolete. In the abstract, numbers stretch on to infinity with no end to how high they can go; however, the largest number that has a name in the English language is googleplex.
Googleplex is built on the number googol, which is 10100 or 10 followed by 100 zeroes. Googleplex is 1 followed by a googol of zeroes. However, this represents a naming convention only. Googleplex plus 1 is a still higher number.
The ongoing search for the highest prime number has produced figures in excess of a googleplex. The Mersenne Prime Project builds on the work of seventeenth century French scholar Marin Mersenne, who discovered a formula for calculating prime numbers. Before his time, the discovery of prime numbers came by chance and prime numbers that fall outside his formula are still slow to be discovered. Mersenne's formula — 2n-1 — has been used to discover many new prime numbers. In 2013, a professor at the University of Central Missouri discovered the 48th Mersenne prime which is 257,885,161-1, a number with 17,425,170 digits.