A wormhole is an astrological object that allows for the easy transport from one location to another much farther away. There was a wormhole near the planet Sakaar that connected to space by a uninhabited, though it could have been, planet that was relatively close to Earth. This wormhole bombarded those inside with gamma radiation. The one near Sakaar was called the Great Portal by Korg and the Kronans.
After the Kronans Korg and Margus fled from Earth they went through the Great Portal but were chased by Beta Ray Bill. When the Illuminati banished Hulk from Earth he damaged the S.H.I.E.L.D. Space Shuttle and went through the portal and crashed on Sakaar. After Bill freed the gladiators on Sakaar he went back through the portal to go back to Earth.