The Sentry is connected to the same energy that powers Captain Universe
Captain Universe is an anthropomorphization of the Unipower - an aspect of the Enigma Force, which is itself an energy field that dwarfs even that of the Power Cosmic in terms of sheer power - and one of the main purposes of the UniPower is to keep the Whirldemons at bay and from crossing over into reality and causing havoc.
The Golden Sentry Serum that Robert Reynolds ingested somehow connected him to the Enigma Force directly (rather than just a piece of it like the UniPower), giving him his near-limitless power, but also acting as a gateway for Whirldemons, which is where his "Void" persona comes from.
