The galaxy contains some four hundred thousand million stars of various types. Of these only a fraction are presumed to have habitable planetary systems, and only a fraction of these have been investigated. Most are situated within the spiral arms between ten and forty thousand light years from the galactic center.
The very size of the galaxy means that, despite the use of faster than light warp drives, most of it remains unknown. Even the human controlled Imperium, by far the largest and most widely distributed of all stellar empires, contains only a tiny fraction of the galaxy's stars. New worlds are constantly being discovered and investigated, along with their attendant civilizations, creatures and resources. Even so, there is no possibility of either humans or aliens exhausting the galaxy's potential to provide new worlds for habitation and exploitation.
Warp space is the medium through which faster-than-light spacecraft travel between the stars. It is, in a sense, an alternate reality or parallel dimension in which the laws of time and space are different from those of our own universe. Movement within warp space bears a distinct relationship to distance traveled in normal space, and this relationship can be manipulated to make faster-than-light travel possible. It is not strictly true to say that distances in warp space are 'shrunk' compared to those of normal space. A more accurate analogy would be to think of warp space as a dense fluid medium which is subject to constant movement, currents, undertows, etc. This is not perceptible in warp space itself of course, because the fluidity is only relative to our own reality. A spacecraft can exploit this phenomenon by entering warp space, allowing itself to be shifted along by its natural flow, and then re-entering normal space a distance away from the starting point. A metaphor commonly used to explain how warping works is that of the fast flowing stream. The stream represents warp space, moving rapidly along its motionless banks, representing real space. A leaf dropped into the water upstream will move along, floating on the surface of the water. The leaf does not move relative to the water, but is merely carried by it until it lodges at some point downstream from its original location. This is a useful metaphor as far as it goes, but it must be remembered warp space is far more complex in its movements than the linear stream, for it can move in all sorts of convoluted and bewildering patterns. Spacecraft are also able to make corrective movements in warp space and can enter or leave warp space at a chosen moment. Even so, warp travel is never totally predictable, either in its duration or eventual destination.
Warp Storms
Warp space is an extremely volatile medium, and can represent a dangerous one for spacecraft within it. Occasionally, the normal current movements of warp space become amplified into raging storms of savage and destructive ferocity. Such storms may last for only a few moments, or they may last for many years. At best, a warp storm might throw a ship off course or delay it, at worst a warp storm can make warp travel impossible in some parts of the galaxy. Storms are constantly forming and dying down, at any time at least 10% of the galaxy's solar systems will be inaccessible because of storms. Half of these systems are cut off for less than a year, but many remain isolated for many years or even centuries. Indeed, some systems have always been isolated, and show no sign of becoming otherwise.
The Human Navigator
Unlike the closely packed empires of other races, the Imperium is flung wide across the entire galaxy, its worlds are often hundreds if not thousands of light years distant. Normally it would be impossible to maintain such a vast area of space as a single political entity. What makes it possible to do so is the existence of human Navigators. Navigators are a sub-species of humanity some of whom resemble humans so closely that they are indistinguishable, others are so physically alien that the relationship is hardly apparent. All navigators are capable of entering a trance-like dream state in which they are able to mentally steer a spacecraft through the medium of warp space.
Under the intuitive guidance of the Navigator, a ship is able to traverse distances of tens of thousands of light years in a single jump. Perceived journey time is 1-4 days per thousand light years, equivalent to 1-6 months of real time. Even so, a journey from one edge of the galaxy to the other would take between 85 and 510 months of real time. For these reasons, worlds remain self-governing even within the Imperium.
The Astronomican
In order to guide their spacecraft through warp space, Navigators require a signal to steer by; a sort of real space reference point which can be perceived from warp space. As only psychic signals penetrate both real and warp space, this signal has to be a psychic one. Some psychics are capable of broadcasting a short range signal of this type (10 light years) but the principal signal is centered upon Earth and is called the Astronomican. Further details about the Astronomican and the psykers who maintain it (the Adeptus Astronomica) are discussed later. For the moment it is only important to bear in mind that the Astronomican permits navigators to utilize their powers. The range of the Astronomican is far greater than 10 light years although it is not infinite. Warp storm activity can also affect the total range, but about 50 thousand light years is the usual distance. As the galaxy has a diameter of about 85 thousand light years, with Earth approximately 30 thousand fight years from the center in the galactic west, this means that the Astronomican does not cover the eastern fringe of the galaxy at all. The Astronomican marks the effective boundaries of human space: human groups existing beyond it are rare, isolated and comprise an unknown quantity.
From Warhammer 40,000: Rogue Trader