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 form 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, Mister Strange?"

- The Ancient One to Stephen Strange

The Multiverse is the collective of 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
Humans, during the 1940's, 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.

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

- Ancient One to Doctor Strange

To be added

Earth Plane
The events of the entire Marvel Cinematic Universe exist within a single and unitary universe, or dimension without a formal name. This regular universe, or as sometimes referred to as the Earth Plane, is often the target of inter-dimensional beings, as it houses a vast range of matter and life that entices many creatures.

It's important to note that the Nine Realms all exist within the same, regular universe. As Yggdrasil, the World Tree, merely acts as means to teleport between nine separate planets separated only by physical distance and not dimensions. In this context, realm does not mean universe.

Quantum Realm
The Quantum Realm is by far the closest realm to the regular universe, as it can and has been accessed through means of decreasing ones size to the extent they enter a different realm.

Even though time and space behave differently in this realm, it's difficult to determine at what subatomic level, that matter leaves the regular universe and enters the Quantum Realm.

Astral Dimension
The Astral Dimension is a parallel dimension in which the astral form of organisms exist purely detached from our physical bodies. In a simplified way, the Astral Dimension contains the 'soul' while the regular universe merely contains the physical form of organisms.

This parallel universe is incredibly close to the regular universe, as some actions made toward the individual within the Earth Plane can have dramatic effects on their form within the Astral Dimension.

Mirror Dimension
Similar to the Astral Dimension, the Mirror Dimension is an exact parallel dimension as to the regular universe. It shows all movement and action that occur within the regular universe, and yet no action within the Mirror Dimension can ever be witnessed nor will ever have an effect on the regular universe. In this way, the Mirror Dimension is used as a battleground away from the regular universe, in order to keep the public in safe.

Dark Dimension
The Dark Dimension is a very unique universe. Dormammu, a large titan-like creature, asserts full control over his universe, and feeds of the planets and stars from the regular universe. Unlike any other dimension, time doesn't exist in the same form that it does in all other universes. In other words, time has no effect in this dimension, hence all aspects of the Dark Dimension remain immortal, including Dormammu.

Individuals have the capacity to draw minimal amounts of energy from the Dark Dimension, energy unique to that universe, in order to immortalize themselves. Yet in this way, it brings the entirety of the Dark Dimension, including Dormammu, closer to our regular dimension.

Hell
To be added

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.

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.