The celebrated astrophysicist (and UFO skeptic!) explained something remarkable a couple of weeks ago during a long-form conversation with Jordan Peterson.
If we wind the clock back to 1920, and look through the then-most-powerful telescopes with the then-most prominent cosmologists, we will conclude as they did that our galaxy -- the Milky Way -- is the universe. Everything that we see is within it and, presumptively, there is only empty space beyond it.
It was Edwin Hubble who disabused the world of that notion. Those smudges way out there? They are separate galaxies, each containing billions of stars, and there are billions of such galaxies! This discovery, made during my father's lifetime, was a putting of man in his place on the same sort of plane as the work of Galileo and Copernicus.
But now we know that the universe is expanding, and that because of the forces associated with the lately-discovered "dark energy," that expansion is accelerating. As a consequence, more and more of the outer universe (from the perspective of Earth) is receding from our view, because the light that it emits, while it is coming towards us at the speed of light, is drawing away from us as well at speeds greater than light, as part of that accelerating expansion.
So, Krauss explains, there will come a time -- a time whose distance in time from 2021 we can measure -- 5000 years? -- when, looking through the most sophisticated telescopes and other sensing devices of that time, we will see only what is in the Milky Way, just as in 1920.
Of course we assume that there will be some history at that point in the future that allows people to remember what is out there; Krauss's point is not that it will truly be forgotten. Rather, his point is that this phenomenon proves that just because we now have a horizon beyond which we see nothing tells us nothing about what is beyond our vision. We simply do not know and cannot know.