The Outside is the Infinite surrounding nothingness outside the Omniverse and the Final omniverse. This and the The noname verse is all that is outside the Final Omniverse, unless you bother to count The Box (which, strictly speaking, isn't necessary since it's assumed it's inside the Box anyway). There is also the set of everything that could exist even if it is not in the box but that is too confusing.
Thanos was cast outside the Omniverse in an issue from March 2016, where he found Love in the form of nothing.
As the Marvel Wiki says: " When the Ultimates went to the outside to try to fix time, Galactus, now a lifebringer, rescued them before the pressure of the great, dark nothing could crush them. In his way back, Galactus opened a portal back into Eternity, which Thanos used to return into the Omniverse, where he would prove his love for nothing"