The concept of a positron, a sister particle to the electron that fluctuates in and out of existence, was new to me. I know this is reductive, but Krauss essentially seems to be saying that if we can observe particles dropping in and out of reality, then it's not a leap to assume that the universe is inherently unstable and that on large scales these tiny whack-a-mole particles accrue into something substantial. Empty space is certainly not a "nothing" in the sense that we can measure its dimensions and draw solid conclusions about it as a medium through which light, gravity, and dark energy propagate. To say that empty space, a vacuum, is nothing, and that therefore something profoundly different, perhaps supernatural happened to create matter within it is, as Krauss shows, an anthropocentric fantasy contradicted by the facts. Along the way he gives an exhilarating summary of the last 100 years of cosmology and particle physics.