system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Superstring Theory.

I am currently reading Brian Greene's book which is good, but if someone can break it down in even simpler terms, share. I am talking "in a nutshell" kind of thing. I have no particular focus (though if you want to discuss quantum mechanics and/or general reality and their roles in SST, please do).
gpt
Superstring theory is the union of string theory and supersymmetry.

Supersymmetry (which is a scientific theory) holds that the Standard Model of particle physics is only a *partial* model. The idea is that there is at least one more symmetry in the model that gives rise to a whole family tree of fundamental interactions we haven't observed yet, because we didn't know to look for them. Supersymmetry hasn't yet been confirmed by experiment — or even really *hinted at* by experiment — but the existence of dark matter tells us *for a fact* that the Standard Model is incomplete, and supersymmetry provides an easy and internally consistent theoretical framework to start exploring.

String theory (which is not a scientific theory) is a family of mathematical methods for modeling the fundamental interactions in nature. It doesn't say anything physical about the universe; instead, it's a set of alternative mathematical formulations. The idea goes that if we can find other, different mathematical models that describe reality as we've observed it so far, maybe those models will *imply* new truths that we can go out and look for.

Supersymmetry is what you get when you combine both approaches: using new and speculative mathematical models to try to predict the characteristics of types of interactions currently not described by the Standard Model.