Rafi Letzter at LiveScience tells us it is called M77232917 (represented in digits, in art, below):

Its only factors are itself and the number 1. That’s what makes it prime.

So how big is this number? A full 23,249,425 digits long — nearly 1 million digits longer than the previous record holder. If someone started writing it down, 1,000 digits a day, today (Jan. 8), they would finish on Sept. 19, 2081, according to some back-of-the-napkin calculations at Live Science.

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Primes that are one less than a power of 2 belong to a special class, called Mersenne primes. The smallest Mersenne prime is 3, because it’s prime and also one less than 2 times 2. Seven is also a Mersenne prime: 2 times 2 times 2 minus 1. The next Mersenne prime is 31 — or 2^5-1.

This Mersenne prime, 2^77,232,917-1, turned up in the Great Internet Mersenne Primes Search (GIMPS) — a massive collaborative project involving computers all over the world — in late December 2017.More.

Prime numbers get really long and thin way up there. Here’s a bit from a page of curious information about various prime numbers: Choose a prime number greater than 3. Multiply it by itself and add 14. If the result is divided by 12, then the remainder will always be 3. More.

And if you have all afternoon, try one of these conundrums:

Is zero even?Absolute zero proven mathematically impossible?

Is celeb number pi a “normal” number? Not normal. And things get worse. Surely this oddity is related in some way to the unreasonable effectiveness of mathematics.

and

Must we understand “nothing” to understand physics?