Hyperspace is a dimension of the multiverse that unfolded in a different way that realspace, the reality of earth and other planets. It has 5 spacial dimensions, and c in E=mc2 doesn't exist in the same way as it does in realspace. The maximum speed that has been observed within hyperspace is c34^5. That is 299792458 metres per second, times 34 to the power of 5. This gives a total value of 1.3621197e+16 meters per second, or 13 621 197 000 000 000 meters every second. However, this speed was observed due to fatal error that resulted in the entire crew of the ship dying due to a warp core gone horribly wrong. In practice, the highest speed ever achieved was c15000 (kc15), allowing the crew to travel from one end of the milky way galaxy to the other in under a decade. It is reccomended that kc20 (c20000) is never exceeded, as even the most powerful of ships may break apart under the stress.
Physics of hyperspace
There are five spacial dimensions within hyperspace, and though a three dimensional ship cannot access them, the creatures within hyperspace can. This leads to incredibly bizzare creatures that seem to phase through reality, or move in impossible ways. In reality, it's simply moving in a way that humans cannot percieve. This reality makes the experience no less chilling to witness.
Matter from either realspace or hyperspace can travel through without harm. It's unknown what allows for a hyperspace material (that is, one that exists naturally in hyperspace only but can be brought into realspace) to move faster than light in realspace, when a realspace material must slow down before re-entering realspace.
Should a realspace ship re-enter realspace, even at c2, the excess energy will be transformed into thermal energy. There have been reported cases of a ship entering realspace at c15, and when people go to find the wreckage, all they encounter is a massive cloud of hydrogen gas moving at near-lightspeed. The force can cause atoms to totally break apart, leaving a soup of sub-atomic particles that collapse back into hydrogen gas.
Hyperphotons are photons that travel at superluminal speeds, and can maintain that speed in both hyperspace and realspace. Whenever a rift is created, hyperphotons spill out, allowing any and all hyperspace rifts being able to be detected. It is unknown how fast a hyperphoton actually travels, therefore, it is currently listed as c∞. Hyperphotons are not the only superluminal particle in hyperspace, but they are among the most common. Some creatures within hyperspace have used hyperphotons to construct a form of nervous system, allowing them to reach impossibly vast sizes.
Hyperphotons are also used to communicate across interstellar distances, using binary code to represent the message being sent.
Temporary rifts are created by a hyperdrive, which uses vast amounts of energy to "punch" a hole through the layers of both universes, allowing travel through. After entering, the hyperdrive is turned off, causing the universes to suddenly snap back together. This produces a massive amount of energy from hypermatter flowing into realspace, and from realmatter going into hyperspace. Therefore, it can be seen if a ship is entering or leaving hyperspace at any time. These rifts are small, often no larger than needed for the individual ship to pass.
Permanant rifts are formed when a hyperdrive suffers from a catastrophic failure. The hole punched between hyper and realspace is one that cannot be closed, leaving a permanent "gap" from which the two worlds can interact. The area around the rift is thick with hydrogen gas from items "skipping" across and hitting velocities that cannot exist in realspace. This gas is heated, and sometimes resembles a small nebula. The rift itself may have objects orbiting it, adrift ships or planets and moons. There's even been recorded to be an earthlike planet that orbits a permanent rift, though it was stripped of all life due to the incredible radiation that existed.
These rifts are known as 'hellgates' in casual speech, but in scientific terms are more properly labelled as HSDFERs, or HyperSpace Drive Failure End Results.
Denizens of hyperspace
Hyperspace is sometimes crudely described as "nightmares shitting nightmares" due to the massive beasts that live there. Though decidedly apathetic and uninterested in actively attacking those who enter hyperspace, many incidents have occured in which a ship goes adrift, and is destroyed by the creatures purely by accident. There has been at least a dozen cases of a ship slipping inside a monster, where immune processes that are utterly alien to any xenobiologist come and attack it, but to date, malicious intention is not found in the creatures of hyperspace.
The scale of these creatures cannot be properly comprehended. People have passed through what they thought to be a cluster of animals...until it blinked, revealing itself to be a planet-sized eye. Some monsters are so large that it is possible for a ship travelling through hyperspace to go through the animal, between cells (or rather the creature's equal thereof) without harming the thing, then emerging from the other side.