Hyperspace is a mysterious dimensional plane that allows humans (and other races) to travel with faster than light speed, being the one and only thing that makes creating interstellar empires possible. The word 'mysterious' isn't used without a reason, since even despite using it for hundreds of years, there exist not a single theory that explains how exactly it is working. With Hyperspace Drive not fitting into any scientific theory to ever exist. What remains, are speculations and conspiracy theories. Further fulled by extreme rate of suicides and insanity among hyperspatial physicists. Which can be considered a sort of tradition of hyperspace, dating back to mankind's first flight in hyperspace, with all people on a Curiosity's bridge returning slightly... different. What allows it to be use as a mean of a FTL travel is that it's space is 'compressed' when compared to one in the Realspace. 1ly in Hyperspace is an equivalent of 1,5 to 500 ly, with compression rate of 70-200 making up for 95% of space. Because of that, a ship has to travel through only a fraction of the actual distance of travel.
Travelling through Hyperspace
Even describing hyperspace is a problem, since every person to travel through it sees it slightly different. Sometimes it a feast of ever shifting colors, sometimes a singular color spread to every point in space, and sometimes completely darkness, with not even a tiniest ammount of light visible in many light years of space. The biggest problem of trying to observe it, is the fact that it isn't exactly healthy for mind. Staring into Hyperspace for too long might cause inrreperable mental disorders. Which is why all sentient species attempt to avoid being exposed to raw hyperspace (non-raw being one processed by sensors and computers), with human prefering modification to the visual sensors of their space suit to filter everything with potential negative effects on their sanity. This method is, however, still imperfect, capable only of minimalizing the chance to catch some incurable mental disorders, that's why using it is a last resort. That was the biggest problem of seeing now... and now the biggest problem of traveling through it. Hyperspace is spatially unstable. Which allows to use it as a FTL drive, while also granting massive and nearly crippling disadvantages, especially debilitating during first generation of hyperspace drives. Distance from Solar System to Alfa Centauri is more or less the same in Realspace (with all changes easy to predict). In hyperspace it's different. The flight between two systems might take for example from three to seven days, simply due - as it was called - spatial drift. Early hyperspace travel required exiting it multiple times to determine your position in Realspace, before attempting to reach the target once again. This flaw was partially fixed with second generation hyperspace drives. A transmitters capable of transmitting signals through hyperspace for much greater distance were created, that could also transmit signals from Realspace into Hyperspace. It allowed setting hyperspace routes from system to system by setting a chain of transmitters in Realspace between them. It allowed much faster and safer transit, while avoiding stationary threats by creating routes around it. All of this while still being able to travel through uncharted lands, just with all the risks and hardships of the first generation hyperdrives remaining. Second biggest problem of hyperspace travel is the need to reach the very edge of star's gravitational influence. For Solar System it requires flying all the way to Kuiper Belt, quite a bit beyond even Pluto. The only exceptions are points of gravitational disturbance like hyperspace planet, and even there entering or exiting hyperspace isn't too comfortable. What's more a ship also has to deccelerate to enter hyperspace, which makes all interstellar travels really slow and costly.
Landscape and Climate
The Hyperspace has its own landscape, the term often summed up to hyperscape. Of course, with Hyperspace being what it is, it's something a bit different than what passes for landscape in Realspace. There is no solid matter involved. The landscape is determined by changes in spatial compression. Each of major hypergraphic features receives a name, typically from the name of its discoverer (or from a name of the ship that fell victim to that particular area. There is also a number of other naming conventions that should be talked through separately. Here however, it has to be mentioned that all such areas are 3D in nature, and are typically rather important for travel and trade.
List of Common Hypergraphic Features
Low Compression - Stable areas with compression rate of 70-100 are typically called 'Seas' and 'Oceans' (depending on the size). They are not exactly helpful for long-distance space traffic, however they have one very important positive side: their stability. While relatively slow, they are also very safe to travel through, and their high visibility offers wide range of detection of incoming threats. And it's not like they are 'extremely' slow. Very Low Compression - Areas with a compression rate of 1,5 to 70, typically called 'Barrens' or 'Wastelands'. They are quite rare, and offer very little (If not zero) possibilities of expansion. Travel there is very slow. though detection range is typically very long. There are also recurring talks of ships disappearing there, with some tales claiming that it's so close to Realspace that something from the depths of the interstellar void sometimes... 'snatches' ships from the Barrens. Stable Changes in Compression - Areas where the spatial compression grows in a stable way. This is pretty important for traffic, because sudden rises and drops in spatial compression are dangerous, especially to non-military ships. When longer they are often called 'Ascends' or 'Descends', while shorter are often called 'Gates'. Very High Compression - Areas where the spatial compression rate goes beyond 200, often called 'Depths', 'Rifts' or 'Trenches'. At this point they stop being useful for traffic, because the visibility is limited, which makes establishing trade routes impossible. Besides, emerging from Hyperspace is downright suicidal in those areas. Dots of High/Low Compression - A relatively normal area characterized with dots of much higher or lower compression. Typically called 'Reefs'. High danger of crashing into wall of vastly different compression with high speed, which generaly is lethal. This forces ships travelling through such a region to go slooowly. Spatial Vortex - A Hyperspace equivalent of black holes. Areas where something is actively pulling things to it, the closer they are to the center the stronger the pull. Quite soon it becomes too strong to fight against, even with modern engines. Vortexes used to be classified as Hyperspace Anomalies, until they become a class of their own. Hyperspace Anomalies - Area where Hyperspace is... disturbed. Spatial anomalies are typically characterized as areas where the hyperscape is naturally unstable. The spatial compression is typically high to very high, but it can undergo rapid changes that follow no established rules. They are extremely dangerous to any hyperspace traffic. However, they seem to be a quite often source of archeotechs.
Hyperspace also has its own weather patterns. On one side there are hyperspace storms, typically erupting from spatial anomalies. They are a raging fronts of spatial distortions, tearing ships apart and essentially changing the Hyperspace travel into a suicidal endeavour. They vary in intensity though. Many storms aren't even considered 'storms', and are instead just a waves of 'bad' hyperspace weather, lowering detection range and causing small damages to weakly constructed vessels. Much rarer is the opposite situation. While moments of relative calmness make for a majority of time in the Hyperspace, true Hyperspace silence happens very rarely. When it does, most of the hypergraphic features are typically replaced with nothing but endless low spatial compression. The stronger the silence, the lower the compression, and the more time it takes for the hypergraphic features to return to the norm.