Hyperspace is a domain of subspace coexisting physically with normal space but within a different layer or level of subspace. The region is largely devoid of matter mostly consisting of swirling red-hued energy eddies and currents in all directions. These currents pull or push vessels along them requiring ships to maintain their course through continuous use of maneuvering thrusters. Since there are no stellar bodies within hyperspace this makes navigation very difficult as it is impossible to tell, without a point of reference, the relative course, speed, and location of a vessel. There is no "magnetic north" within hyperspace to base navigation on.
Relation to Normal Space
Hyperspace is coupled to normal space as with any subspace domain but the correspondence is not 1:1, rather distance in hyperspace is smaller than the relative distance in real space, or rather hyperspace is compressed space. Depending on the precise conditions, one kilometer traveled in hyperspace can be many hundreds of times more kilometers distance traveled in real space. As such a vessel traveling at conventional sub-light speeds in hyperspace can achieve relative FTL travel in real space without ever going faster than light.
On average the relationship between distance travelled in Hyperspace with distance travelled in real space is about equivalent to Warp 9, so right around 1500 times the speed of light. This varies, however, due to gravitational "shadows" of real space phenomena in Hyperspace. Gravitational objects in real space that are super-heavy (stars, black holes, and so on) also impact the environment within Hyperspace and this is the cause of the eddies and currents that make Hyperspace so dangerous. High gravity objects in real space compress the relationship between Hyperspace and real space more, thus going near a black hole in its corresponding point in Hyperspace would technically allow for faster travel but the currents of Hyperspace would be stronger and thus be more dangerous to traverse.
Hyperspace Weather
Automated probes are currently being used by the Vulcan Science Academy to attempt to derive weather patterns in hyperspace and map out the real state of the currents of energy which push and pull at vessels in hyperspace. These probes are also attempting to plot the conditions by which distance in hyperspace relates to distance in real space through the use of short-range tachyon beams and vessels within real space.
So far the project has had only limited results.
Sensing / Communicating
Although transmissions can breech the subspace boundaries, such as with the beacon network, sensors cannot. It is impossible to sense real space from within hyperspace or vise versa. Since the coexist in the same 3 dimensional realm but at different layers there is a direct correspondence of a location in real space to every location in hyperspace. Only a tachyon beam transmission can penetrate the divide between normal space and hyperspace.
Signals from Hyperspace distort rather quickly compared to range. This is one of the things that can make travel through Hyperspace so dangerous. Jump Gates use powerful tachyon beams to allow vessels to find them. This allows a vessel to get a lock on a specific signal. Since signals become distorted so easily within Hyperspace a positive lock is important to maintain at all times. Navigators will use multiple Jump Gates as navigational references to their final destination in order to maintain a positive lock on at least one Jump Gate at all times. This allows vessels to always have a fix on an exit port back into normal space.
The Rim
The rim of known space is roughly defined as the edge at which a vessel in hyperspace can still detect the navigational beacons within hyperspace. Beyond this barrier vessels have no way to navigate back to known jump gates. The Rim is essentially the boundary of known space as detectable from within hyperspace.
Jump Gates / Jump Points
Transit into hyperspace is only possible from a jump gate or a jump point capable vessel. Vessels capable of forming a jump point create a temporary vortex which allows entry into hyperspace, due to the size and power requirements, they are rarely used as it takes more power to create a jump point than it does to use warp drive and the two technologies are incompatible to be used at the same time. A warp drive system would be required to generate the power to create a jump point where as it would have to be shut down prior to entering the jump point.
To allow for navigation specially designed subspace beacons in normal-space broadcast within hyperspace. These signals use tachyon beams between beacons jump gates and the coordination of tachyon beams / beacons allows vessels to navigate through hyperspace safely through the beacon network.
Without these beacons a vessel within hyperspace would be unable to navigate.
To aid in navigation many beacons on non-jump gates exist at planetary facilities, space stations, and even very large vessels to assist in navigation.
Most ships that travel in hyperspace have a built in fail-safe that automatically shuts down the sub-light engines if the computer fails to receive a given command during a predetermined period of time while in hyperspace. This guards against a ship drifting too far off the beacon in the event that the crew are incapacitated or killed and increases the chances of the ship being found and rescued by a passing vessel.
Side Effects
Hyperspace Travel Syndrome or "HTS" is a common affliction among long range interstellar pilots. It is caused by long term exposure to the conditions of hyperspace in which the view outside the cockpit windows of a space craft gives the pilot the impression that the apparent motion of the craft is not the actual motion as the craft’s instruments would indicate. This combined with the disorienting effects of the constantly flickering and shifting patterns of light and false shadow over time begin to play tricks on the brain, scrambling perceptions of up and down, forward and backward. Some pilots even say it's like being caught in a dream or nightmare from which one could not awaken. Symptoms include nausea, vertigo, panic attacks, and an overwhelming feeling of motionlessness.
It is because of HTS that many passenger ships do not have windows in the passenger compartments, or close them during travel through hyperspace. A pilot, however, cannot risk flying blind given the dangers inherent in hyperspace travel so it is necessary for a ship’s flight crew to learn to live with the side effects, train their eyes to see past the optical illusions and trust their instruments. Hyperspace has also been found to enhance and/or alter certain mental abilities, most notably those in the psionic category, although what causes this phenomenon is still unknown.
Regulations
Due to potential mental issues from long exposure to hyperspace conditions, it is illegal for anyone to be forced to stay in hyperspace for extended periods of time against their will. Additionally, any combat, for any reason except for self defense against an aggressor who has already broken this law, is illegal.
Hyperspace Combat
Engagements in hyperspace are rare and most often avoided by all parties as such battles have often turned out to be disasters in the past. Aside from the dangers inherent with hyperspace travel, the unpredictable spatial knots and gravitational inclines can cause missiles and even energy weapons to bend off course, either missing the target entirely or even strike a friendly ship. As such, what little fighting in hyperspace does take place is usually one-on-one over a very short range and often strictly hit-and-run strikes. Sometimes using small groups of fighters and/or smaller warships as large fleet engagements are out of the question.