What requires the fewest assumptions?

That...

a single, functional, self-sustaining human brain fluctuated into existence about 50 years ago (when my consciousness began)

...or that...

an unimaginably enormous and complex universe fluctuated int existence billions of years ago, and through a nearly endless series of unlikely events, expanded to allow billions of functional human brains to evolve on a planet which just happens to be perfectly suited and situated to sustain them.

What I find scary about this Boltzmann Brain idea is that there wouldn't have to be more than one... me!
...and that Occam's Razor seems to imply that that singular delusional existence is most likely true.

Sorry fellow readers... you don't exist!