Wormhole

"There is the possibility that this could be the precursor to the opening of an Einstein-Rosen bridge." "A what-what bridge?" "An Einstein-Rosen bridge, also known as a wormhole. In layman's terms, a portal to another universe opening below the ozone layer."

- Phil Coulson and Nick Fury

A Wormhole (also known as an Einstein-Rosen Bridge) is a special portal or tunnel created to quickly travel from one place to another, no matter the distance between them. Most wormholes in the known universe were created by the Bifrost Bridge or the Tesseract. In physics, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime.

History
"A hostile alien army came charging through a hole in space; we're standing three hundred feet below it."

- Tony Stark

Thor and other Asgardians used the Bifrost Bridge to make wormholes to travel to other places. In 1945, when Johann Schmidt took the Tesseract with his bare hand, he seemed to be transported by a wormhole to an unknown location. At the beginning of the 21st century, Jane Foster was researching wormholes and studied the possibility of them.

When the Avengers battled the Chitauri in the Battle of New York, the invaders came to Earth through a wormhole made by the Tesseract.

During the 2013 Convergence, many wormholes opened up around the Nine Realms, all coming together in Greenwich. Malekith attempted to use the Aether to destroy reality with these wormholes, but was stopped by Thor, Darcy Lewis, Jane Foster, and Erik Selvig.

Since ancient times, the group that would later be known as HYDRA kept a Kree artifact: a Monolith that was originally intended to defeat Inhumans, and regularly sent human sacrifices through it to another planet to appease an exiled but powerful Inhuman. During the War against the Inhumans, the Inhumans become aware that S.H.I.E.L.D. had the Monolith and attempted to recover and destroy it. After the war, S.H.I.E.L.D. kept it from the crippled Inhuman population of Afterlife. Agent Jemma Simmons was swallowed by the Monolith, quickly prompting her partner Leo Fitz to track down every myth and every possibility surrounding the Monolith. Eventually with the help of the Asgardian Elliot Randolph, S.H.I.E.L.D. managed to retrieve Jemma Simmons and caused the stone to explode. However, Gideon Malick procured different fragments and, using the instruments within an English castle, createx a wormhole for Grant Ward to retrieve It. Chasing Ward, Phil Coulson entered the Wormhole also. Ultimately, S.H.I.E.L.D. led an Attack on the HYDRA Castle, retrieved Coulson and Leo Fitz, who was also sent through the Wormhole to Maveth, and destroyed the castle; however, It traveled through the Wormhole, using the corpse of Ward.