So on the latest Kurzgezagt video (I know, I know, their channel sucks, that's not the point), they mention one point that feels relevant. They talk about how due to expansion, in some billion years, all stars and galaxies outside the local cluster will not be visible to us anymore, and the sky is going to look basically completely dark for the rest of eternity.

They mention that if a civilization started at that point, with no previous knowledge, they would never be able to figure out that there was a universe beyond the local cluster, because to them it would be basically "unobservable universe" at some point.

Is this true? Would there really be no way? And if so, what are the things that are currently inaccessible to us regardless of how far science goes?