The theory of Boltzmann brains is that you're way more likely to get just a brain (including false memories of a planet earth and a whole visibile universe around it), than to get a brain, a planet earth, and a whole visible universe around it. So the chance that any of that is real, given that a brain exists to perceive it, is infinitesimal. We are probably just floating human brains that popped into the vastness of space three microseconds ago, complete with false memories of the distant past.

To dispel a misconception: They're not some hypothetical type of brain that exists as pure quantum fluctuations (though those are even more likely). Boltzmann was talking about the probability of actual flesh-and-blood human brains arising spontaneously out of the vacuum.