The talk partly overlaps with and reiterates material from Stuart Armstrong's presentation, Von Neumann probes and Dyson spheres: what exploratory engineering can tell us about the Fermi paradox at the Future of Humanity Institute (highly recommended!).
The Fermi paradox is the contrast between the high estimate of the likelihood of extraterritorial civilizations, and the lack of visible evidence of them. But what sort of evidence should we expect to see? This is what exploratory engineering can tell us, giving us estimates of what kind of cosmic structures are plausibly constructable by advanced civilizations, and what traces they would leave. Based on our current knowledge, it seems that it would be easy for such a civilization to rapidly occupy vast swathes of the universe in a visible fashion. There are game-theoretic reasons to suppose that they would do so. This leads to a worsening of the Fermi paradox, reducing the likelihood of "advanced but unseen" civilizations, even in other galaxies.
See also the talk slides.
Anders Sandberg, Where are they?, TEDxUHasselt.