[lloeki]

> a single brain to spontaneously and briefly form in a void

I recall first hearing about this by being introduced to it via the "giant marshmallow hypothesis"†: it's somewhat equally likely that a giant marshmallow spontaneously and briefly form in a void. A Boltzmann brain is then merely a different arrangement of atoms and stuff.

† I don't think it has a specific name.

[marcosdumay]

A giant marshmallow is way more likely. Not all arrangements of atoms have the same probability.