Boltzmann brain is a hypothetical concept in physics that states, given enough time and a universe with a high enough entropy, it is more probable that a self-aware brain would spontaneously arise in the universe by chance than for a complex universe (like ours) to form through natural processes. This brain could be a simple, self-contained entity that thinks it is a full-fledged sapient with or without pre-existing memories, thoughts, and experiences.
The concept challenges the assumption that the universe we observe around us is a result of natural processes, rather than a product of pure chance or a hallucination of a self-contained entity. In our timeline, Boltzmann brain was considered to be an unlikely scenario until the discovery of the enigmatic entities called the Antecedent.