Hyperspace (Realm)

Hyperspace (the place) is the realm objects enter upon exceeding the speed of light. Nothing more is known via observation, other than that it's impossible for any sort of observation to be made. All else that is known is based off of highly theoretical math.

Composition
The prevailing theory on the composition of hyperspace is that it is comprised of particles that not only exceed the speed of light, but also exist in 5 dimensions, making them unobservable to 3D beings. Another proposes that hyperspace is made of variants of the tachyon (a faster-than-light particle) that are in too high an energy state to be observed.