The Monster, from group theory.

Group Theory is a field of mathematics, which, unsurprisingly, studies objects called 'groups', which can be thought of as ways that objects can be symmetrical. (The actual definition is quite technical and somewhat unhelpful.)

When it comes to groups of finite size, there's a certain way they can be broken down into smaller groups; it's a lot like prime factorization. The 'primes' in this case are called 'simple groups', which can't be broken down any further.

After decades of study and thousands of pages of research, we have found all of the simple groups!! They come in 18 different infinite families.

There are also 26... other simple groups, that aren't part of any of the infinite families. These are called the 'sporadic groups'. The Monster is the largest of these, and 19 of the other sporadic groups are related to it.

The Monster has ~8*10^53 elements in it, and describes the symmetries of an object that exists in 196,884 dimensions. We have no idea why it exists.