Current thinking, with a degree of empirical evidence, is that our universe is bordered by an unknown number of 'parallel' universes, known colloquially as 'Dimensions'.

Transdimensional theory has been primarily furthered by the work of Professor Matik of the University of Marque and Professor Wiseman of Evangelou Polytechnic.

Transdimensional Travel

The process of actually transiting physically between dimensions has so far only been achieved accidentally and never in a controllable way.

Archeologists have theorised that the Forerunners many have had access to the technology to travel inter-dimensionally, evidenced by the putative existence of Forerunner regression colonies encountered during an accidental incursion by Earther military units into 'Universe B' (not its scientific designation).

Additionally, a clearly transdimensional incursion into human space occurred at Gorilla in Q0, by a species known only as 'The Visitors'. There have only been two known incursions into human space from this species.

The Evangelou Incident had a number of humans (including ex-Prime Minister Young of theWolf359 Commonwealth) drawn into, and trapped within, another dimension that might have been consistent with 'Universe B'. However, in return for assistance to return to human space, an agreement was reached with the inhabitants of that Universe that humanity would never again visit that dimension.

There have been many efforts to develop a reliable trans-dimensional transit system to allow controlled movement between dimensions. To date this is proved to be impossible.