Answer :

9999
The largest known prime number is [tex] 2^{57,885,161} {[/tex] -1
The largest(biggest) prime number ever known is 2⁵⁷,⁸⁸⁵,¹⁶¹ -1 " (Its 2 multiplied by itself 57,885,161 times, less one)
This number is said to have 17,425,170 digits!