Basically, the void between galaxies isn't actually that much of a void. It's just in intergalactic space stars tend to be hundreds or thousands of light years apart rather than the 5-10 (where we are in the Milky Way) or, say, in light weeks or less in the galactic center.
It's known (and he talks about) stars being ejected from galaxies. This is important because a lot of intergalactic stars have high metallicity and are likely to at least some of the time still have their planetary systems.
It is interesting to consider where these stars came from. A nearby galaxy being the obvious candidate. I wonder if the 3D velocity measurements can be precise enough to ever determine this to a high degree of likelihood. It would be cool to find some intergalactic visitors in our own galaxy.