You're a fucking Boltzmann brain.

Boltzmann brains fail the Monty Python test. This is what happens when you let clowns like Sean Carroll think that physics is the only science that matters. And bad physics at that.

The argument for Boltzmann Brains is that over a sufficiently long time, random fluctuations could cause particles to spontaneously form literally any structure of any degree of complexity, including a functioning human brain. Jeez guys, did you sleep through biology class? How about some bloody common sense? Without a body, food, water and oxygen, the lifespan of an unprotected brain floating in space is about half a second.

Okay, so then let's cross off the literal human brain thing, and assume that a Boltzmann Brain is some sort of self-powered, vacuum-hardened computational machine that merely imagines it is three pounds of meat with the consistency of tapioca. It's still bullshit, because the basic premise that "over a sufficiently long time" mere random fluctuations can form literally any structure is false. That's based on a really naive view of probability. The more realistic view is that of a random walk through a multi-dimensional design space, that is, in a space with N degrees of freedom for some very large (billions?) value of N. (To be clear, I'm not talking about our 4-D space-time continuum, but about the space of all possible structures made from all possible materials.)

For a 1 or 2 dimensional random walk, it is true that eventually the walk will reach every point. But for 3 or more dimensions, that's not the case. No matter how long you wait, an N-dimensional random walk for N greater than 2 will not reach every part of the space, and the higher the N, the smaller the chances of reaching any specific position. With billions of degrees of freedom, a random walk will only visit a tiny proportion of the design space, so the chances of a Boltzmann Brain spontaneously forming is vanishingly small no matter how long you wait.

TL;DR: we're not Boltzmann Brains.