Boltzmann Brains are theoretical disembodied minds that may pop into existence due to quantum fluctuations. The theory can be hard to follow but basically it says that if given enough time and an infinitely expanding Universe that anything will happen. Since it's easier for the Universe to create a mind than it would be for the Universe to create a new Big Bang and have matter evolve to the point where conscious creatures naturally evolve, then therefore the Universe should at some point create these disembodied minds. More importantly, these minds would become the dominant observers in the Universe and would outnumber any other kind of mind.
It is highly unlikely that such minds exist today. However, toward the end of our Universe when the Universe is mostly nothing but a dark, black void with a thin soup of particles, it is possible for these minds to come into existence. These minds would come in all varieties. Some would be on a lower order than human minds. Some would be on an extremely advanced, god-like level. In fact, if God exists then it may be a Boltzmann Brain, having simply popped into existence out of nothing in between the creation of Universes. However, Boltzmann Brains are even stranger than I've detailed so far.
Given enough time anything is possible. Given infinity anything is guaranteed to happen an infinite number of times. This means that if Boltzmann Brains will exist at some point in the distant future then there will be Boltzmann Brains that have a perfect copy of your memories and experiences. In effect, you, I, and everyone who ever lived would be reincarnated as a one of these entities. There's only a minor drawback. Most of the entities would only exist for a very small length of time before they disappear back into the nothingness from which they came from. However, since anything is possible in infinities, some would at least be able to exist for a good deal of time.
Finally, I would like to propose a couple more bits of weirdness with regard to this subject. First, you might be a Boltzmann Brain. You might think you are reading my words while sitting at your computer in your home but you actually might be a disembodied mind floating through empty space. Everything that you see, hear, feel, touch and taste would simply be an illusion you created. The reason this may be true is that if Boltzmann Brains will exist someday then they would be the most dominant form of intelligence to have ever existed in the entire history of the Universe. Statistically, you and I would stand better chances of being Boltzmann Brains than of being real humans. Second, Some individuals believe that Boltzmann Brains existing in the Multiverse might be capable of interfering with our reality. In fact, some believe the Boltzmann Brains may have created our reality and even control it. The notion of "the gods" of the ancients might be correct.
Don't fret just yet. Right now scientists are trying to come up with a solution to the Boltzmann Brain paradox. If the Universe doesn't expand infinitely then that would most likely prevent Boltzmann Brains from forming. Some scientists also believe that the Higgs particle might actually end up destroying the Universe before Boltzmann Brains can form as well. However, the truth is that we just don't know at this point. So are you human or are you Boltzmann Brain?