The Doctor Strange films present the most esoteric corner of the Marvel Cinematic Universe, constantly throwing around fantastical terms and concepts throughout. The first film put a lot of weighty emphasis on the Dark Dimension, the glowing purple zone in which the final bargain took place and from which the villains gained their power.
The Dark Dimension is actually first referred to in the MCU in an episode of Agent Carter, and it went on to be involved in serIes such as Agents of S.H.I.E.L.D. and Cloak and Dagger. Doctor Strange has the MCU story most deeply connected to the concept, but it remains a fixture of the MCU and regularly appears in unexpected places.
Most of the proper lore delivered about the MCU's Dark Dimension comes to fans through Doctor Strange. In that film, fans learn that there are seemingly infinite alternate dimensions, separated but everpresent. An early scene sees Stephen Strange astral project his consciousness on a mind-bending tour through multiple trippy alternate realities. Tilda Swinton's Ancient One, the sorcerer supreme of the moment illustrates the importance of never visiting the Dark Dimension, explaining that it is home only to the evil lord Dormammu. By the end of the film, Strange is forced to defeat Dormammu by outwitting him and the Dark Dimension is forced from Earth at the last second.
Strange's experience with the Dark Dimension is not the only one explored by the MCU. Cloak of Cloak and Dagger fame is a teenaged boy named Tyrone Johnson who gains superhuman powers after exposure to an industrial accident. Johnson is capable of teleportation, among other gifts, due to his accidental connection to the Dark Dimension. He wields Darkforce, a form of cosmic radiation which emanates from the Dark Dimension, to travel between dimensions and physical space as a result. The series features no mention of Dormammu, the Dark Dimension portrayed here is an unfeeling void, rather than a hostile nightmare.
The Dark Dimension also appears in Agents of S.H.I.E.L.D., wherein it takes yet another different form. This series' Dark Dimension grants power to another person and unveils another piece of its complex lore. Season 4 of the long-running series introduces Robbie Reyes, the current Ghost Rider. Reyes gains a bond with a Spirit of Vengence which turns him into a flame-skulled warrior. The Spirit of Vengence comes forth from the Dark Dimension, now looking a lot more biblical than its previous iterations. Fan-favorite character Phil Coulson is almost trapped in this Hell but is rescued and returned to Earth.
Why does the Dark Dimension keep changing from one work to another in the same universe? The reason is simple, the Dark Dimension is an amalgamation of multiple existing dimensions. Interestingly, this is true both inside and outside of canon. In canon, the Dark Dimension is composed of every reality that Dormammu has conquered. In his endless path of domination, Dormammu has added each of his conquests to his bounty to create a theoretically infinite expanse. Hell is contained within the Dark Dimension, as is the glowing purple space present in Doctor Strange, as is the pitch-black void occasionally glimpsed elsewhere.
Outside of canon, the MCU has diverged from the comics, combining multiple concepts from the sources into one concept for simplicity and conveyance. Marvel Comics has a Hell, a Darkforce Dimension, and a Dark Dimension. Hell is where Ghost Rider's Spirit came from, Cloak gets his power from the Darkforce Dimension and Dormammu resides in the Dark Dimension. These three discrete realities are separate because they were devised by separate writers, in separate series, often decades apart. When it came time to bring these slightly heady concepts to the big screen, the MCU had the clever idea to blur the lines and package these ideas as one.
There is some debate to this, aspects of the fanbase may argue that the multiple dimensions are still separate. But the terms and iconography are effectively interchangeable to the point that the Dark Dimension is a total amalgamation. It contains multitudes and is capable of incredible things. Fans learned from Doctor Strange that through the power of the Dark Dimension, one can obtain eternal life. Beyond that, it can apparently be used to bond with demons or access bizarre forces. Many characters are intrinsically linked to the Dark Dimension through their power sets or backstories.
Ultimately, the Dark Dimension is a perfect source of conflict. Seemingly anything can happen in the frayed conglomeration of realities, and the myriad storytellers of the MCU have used it to bring across many unique stories. The concept was introduced into the comics almost sixty years ago, and today, through some clever combining of concepts, the MCU can simplify what once was complex and give context to many different ideas with a single setting.