Multiverse

"You think you know how the world works? You think that this material universe is all there is? What is real? What mysteries lie beyond the reach of your senses? At the root of existence, mind and matter meet. Thoughts shape reality. This universe is only one of an infinite number. Worlds without end. Some benevolent and life-giving; Others filled with malice and hunger. Dark places, where powers older than time lie... ravenous... and waiting. Who are you in this vast multiverse, Mr. Strange?"

- The Ancient One to Stephen Strange

The Multiverse is the collective term for all parallel dimensions in existence.

Sorcerer Supreme
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.

First Recorded Contact
"Zero matter is always drawing energy into itself. It'll always be the coldest object in the room because it is literally devouring any nearby energy."

- Jason Wilkes

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.

Ant-Man's Experience
"It means that you would enter a Quantum Realm. A reality where all concepts of time and space become irrelevant as you shrink for all eternity."

- Hank Pym to Scott Lang

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.

Attack on the Sanctums
"Ever see that in a gift shop?"

- Ancient One to Doctor Strange

To be added

Trapped Between Dimensions
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.

Parallel Universes
"How could I split the world apart? My powers aren't that strong. I am not that strong." "Maybe not yet, but you will be." "And how do you know that?" "Because planet Earth went from smooth to chunky, and Quake is the one who did it." "And you don't think I'd remember that?" "Well, actually, not according to the Multiverse theory. You probably never heard of it, but in quantum physics, there's a theory that, for every universe, there are infinite parallel universes. So in my universe, you destroyed the planet."

- Quake and Deke

Approximately ninety years after the destruction of planet Earth, a human Deke claimed that the Inhuman Quake was the one who caused that. When she replied that she wasn't 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.

Dimensions

 * Earthly Plane: The universe where Earth is located, alongside the other Realms and a vast number of other planets and galaxies.

Inter-dimensional Travel
Travelling between universes through the Multiverse is incredibly difficult and usually requires a lot of energy. Yet several short cuts do exist. The Masters of the Mystic Arts use Sling Rings to travel between dimensions, as long as they can invoke a clear enough image in their minds as to where they want to go. However, it has been shown that some dimensions can be accessed without the use of the Sling Ring, but only by extremely powerful and knowledgeable people. Notable dimensions that have been accessed without the Sling Ring are the Astral Dimension, Mirror Dimension, Quantum Realm, and the K'un-Lun Dimension.

The Astral Dimension can be accessed by souls through Astral Projection; physical bodies cannot enter that dimension. Similarly, the Mirror Dimension can be accessed without using a Sling Ring; however, one needs a Sling Ring in order to get back to the Earth plane, otherwise they get stuck in the Mirror Dimension.

Others have also been able to travel between dimensions. Aida, with the help of the Darkhold, constructed an Inter-Dimensional Gate to free Phil Coulson, Leo Fitz and Robbie Reyes from being trapped between two dimensions.

Once in a few years, the gates to the K'un-Lun Dimension opens naturally in the mountains of China and the transition between Earth and the dimension is free.

The Abilisk is a creature capable of crossing between dimensions as well.

The chamber containing the Aether was accessible from London through an inter-dimensional portal during the Convergence.

Behind the Scenes

 * The Multiverse mentioned in Doctor Strange is referring to the collection of dimensions contained within Earth-199999. In the Marvel comics, the term Multiverse refers to the collection of all alternate universes, not simply dimensions within one universe.
 * The possibility of parallel universes was teased in Doctor Strange by Karl Mordo when he warned Strange of the possible consequences of using the Time Stone. It was further teased in A Life Spent by Deke when he told Daisy Johnson about the Multiverse theory.