Multiverse

"Who are you in this vast multiverse, Mister 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

To be added

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.

Dimensions

 * Material Universe: The universe were Earth is located, alongside the other Realms and an infinite number of other planets and galaxies.

Inter-dimensional Travel
Travelling between universes through the Multiverse is incredibly difficult and will usually require 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. The two dimensions that have been accessed without the use of the Sling Ring are the Astral Dimension and Mirror 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 achieved travelled between dimensions. Aida, with the help of the Darkhold constructed and Inter-Dimensional Gate to return Phil Coulson, Leo Fitz and Robbie Reyes to the Material Universe.

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

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.