The Multiverse is the collective term for all parallel dimensions in existence. Agamotto became the first Sorcerer Supreme to detect the presence of other dimensions within the Multiverse. He dedicated his life to protecting the Earth realm from extra-dimensional threats using magic.
During the 1940s, Humans began experimenting with nuclear energy; during one such experiment performed by Isodyne Energy, a rift was created where a substance dubbed Zero Matter entered this dimension. Eventually, the scientists Whitney Frost and Jason Wilkes became infected with the substance. Frost actively sought more of the Zero Matter and reopened the rift; however Wilkes entered the opening. Upon his return, he described his experience in another dimension outside the Earth realm. Hence it's unknown what realm Zero Matter came from, but confirmed it's an inter-dimensional substance.
In 2015, Hank Pym gave Scott Lang the Ant-Man Suit he had used in the past. While on a mission with his wife Janet van Dyne, Pym witnessed his partner shrinking into a subatomic size in order to break through a missile's titanium shell and disable it. Although the mission was a success, van Dyne was lost in the Quantum Realm, another dimension where time and space become obsolete notions. Pym, heavily traumatized by this loss, warned Lang about the danger of shrinking to a too great extent and ordered him never going beyond the suit security boundaries.
However, Lang had to undergo the same experience as Janet when Darren Cross, who had built the Yellowjacket Suit, threatened the life of his daughter Cassie Lang. Lang went to Maggie Lang's House and had no choice but to shrink at a subatomic level in order to break through the titanium of the Yellowjacket Suit to disable it, just like van Dyne had done with the missile. Much like her, Lang succeeded. However, he was in turn trapped in the Quantum Realm. As he traveled deeper and deeper inside the Quantum Realm where all time and space was eternal seemingly inescapable, Ant-Man began desperately searching for a way to return home rather than be trapped within there forever. In his resolution to return to his daughter and survive, Lang retrieved a Pym Particle Disk and placed it into the regulator of his Ant-Man Suit. His attempt to return to normal size was successful and he reunited with Cassie.
When questioned later by Hank Pym about his experience in the Quantum Realm, Lang told him that he did not remember anything, to which Pym hypothesized that the human mind was not fit to undergo such experiences.
On a S.H.I.E.L.D. mission to rescue the supposed 'endangered' Eli Morrow, whom was actually creating a machine powered by Quantum Batteries, a lethal amount of inter-dimensional energy was expelled throughout the Roxxon Power Plant. And this subsequently transported the nearby Leo Fitz, Phil Coulson and Robbie Reyes to a trans-dimensional state between the Material Universe and Hell, the dimension in which they were slowly being dragged further towards. During this time, these three men could see and hear everything that occurred in their universe and yet were unable to communicate and interact with anything.
Having figured out that Coulson, Fitz and Reyes were trapped, Aida, who had begun reading the Darkhold, began constructing an Inter-Dimensional Gate to rescue them. After some time, in which they, Reyes in particular, were being dragged to Hell, Aida finished the portal and brought them back into their universe.
After S.H.I.E.L.D. returned to their own time period, they discovered multiple Monoliths being stored in the Lighthouse. When the white, grey and black Monoliths were destroyed by an explosive Kree Orb, they created a rift to the Fear Dimension which leaked to Earth. This resulted in multiple illusions based on fears to appear and strange shifts in reality to change the atmosphere of the Lighthouse. The illusions created by the rift would haunt the agents of S.H.I.E.L.D.. The only way the rift could be closed or suppressed is with compressed Gravitonium, which was sought out by the team.
Approximately ninety years after the destruction of planet Earth, Deke claimed that the Inhuman Quake was the one who caused that. When she replied that she was not strong enough to cause such a disaster, Deke mentioned a quantum physics theory that for every universe there are infinite parallel universes, implying that she came from the past into the future of an alternate universe and not her own.
Known Dimensions:
Quantum Realm
Darkforce Dimension
Astral Dimension
Mirror Dimension
Dark Dimension
Hell
K'un-Lun Dimension
Fear Dimension
