The Great Portal, as it is known, is a wormhole that randomly connects with different parts of the universe and deposits the traveler on the surface of Sakaar in the Tayo star system. The portal however has the effect of sapping the strength of any individual that travels through it. Sakaaran Imperials have attempted to study the comings and goings of the portal but so far its appearances appear to be random.
In the year 536 Post, during the height of the Spike War which plagued the planet Sakaar for decades, a cosmic vortex opened in the planets orbit. Upon the vortex's first appearance, it emitted a powerful blast of energy that shattered the moon Sabyr and caused massive earthquakes and tidal waves on the planets surface. At first they believed that it was a sign of the end of the world. But over time alien technology began landing on the planet and they began calling it the "Great Portal" But it is believed to be the origin of the Spikes.
Alternate Universe Versions
Marvel Cinematic Universe (Earth-199999)
On Earth-199999, Sakaar is covered by wormholes rather than just having one, and the largest is known as the Devil's Anus.