The theory posits that the Sentience of the Multiverse is seen in The Collector’s lecture, as well as some symbology that echoes the helmet of Galactus.
The theory speculates that one of the planets seen is that of Taa, which just so happens to be the home of Galan.
Galan merged with the essence of the Sentience of the Multiverse, surviving the renewal and becoming Galactus.
That name may not be familiar, but Galan is the humanoid who eventually becomes the Devourer of Worlds, Galactus.