The universe is far from empty... and what it contains is often key to many science fiction plots.
Some Asteroids are planets and moons that did not quite make it. Some are moons that have lost their orbits. Some are the result of a collision between planets or moons. They range in size from the size of golf balls to the same size as a planet. [Armageddon, SW:Empire]
Asteroids are normally found in belts orbiting planets or stars, but the occasional stray asteroid is also common. [Armageddon, SW:Empire, Star Blazers/Yamato]
In general they do not have their own atmospheres, and cannot support life, but there are exceptions. Some space going organisms, while rare, have adapted themselves to living in asteroid fields.[Galaxy Rangers, SW:Empire]
Asteroids are often good candidates for mining operations.
Another strange property of asteroids is that they seem to absorb the energy from ships. [Yamato/Star Blazers, Transformers]
Black holes are stars that have collapsed in on themselves resulting in an intense gravity hole that absorbs everything that comes near, including light itself.
Black Holes are sometimes seen as the entrances to worm holes or some other form of gateway, with the inability to overcome the intense gravity being the only thing preventing their use as a reliable method of interstellar travel.
Galaxies are vast expanses (or maybe clusters) of stars and planets. They are born of great explosions and expand into space... It is assumed that they die when they fall back into themselves. Sometimes they collide.
Galaxies are vast to the point that it seems rare if a story takes place in more than one galaxy... (now that does suggest several story ideas...)
Hyper Space is something different and above real space... Objects traveling within it travel faster than they would in real space. It can be entered by ripping holes or gates in real space and passing through them. It is closely linked to real space, however... planetary bodies that exist within real space have gravity echoes in Hyper-Space that relate to their real-space locations.[B:5, SW]
Hyper-Space does seem to have its own set of physical laws. In most representations of Hyper-Space it even generates its own light, has its own strong gravity currents that make navigation difficult, and has its own life forms living within it. [B:5, Tomorrow People, Honor Harrington Books]
Most of the time combat in Hyper Space is avoided. Due to the strange properties of it that make navigation so difficult, any combat that occurs within Hyper-Space usually turns out badly for all the parties involved.
Magnetic and Ion storms do not have clearly defined causes, and cannot always be predicted, but can sometimes be charted. In general they are a threat to navigation, and able to destroy any ship not strong enough to survive them. In general they are the space equivalent of hurricanes.
Meteor Showers are the result of a collision between large asteroids, moons or planets. They hit, and then shatter spraying meteorites everywhere... If a ship is not properly prepared for them they can do great amounts of damage. The Space equivalent of Hail. [Transformers, SW]
Moons are small planet like objects that orbit other planets. Some like the one orbiting Earth, are barren and lifeless. Others are misshapen like the ones around Mars... While Others are planet sized themselves and able to sustain life. [SW]
Planets are large bodies that orbit stars. While they usually have a molten core, many are made up of rock, and some are made up of gasses. Most planets have at least some minerals or metals that can be mined, others are even more valuable for their ability to support life.
Plasma storms are localized energy storms that have the ability to destroy anything that their energy discharges come into contact with. [The badlands in Trek]
Basically a multiverse is a combination, or a grouping of separate but linked universes. Many of the universes in the multiverse are mirrors of each other, with minor differences between them... like one made up of furry animorphic people instead of humans living on Earth. [see various Marvel comics]
Nebulas are large gas clouds for which the first thing noticed about them is that they are pretty to look at. Some were formed after a star went nova, others will slowly condense into a new star.
The dense nature of nebulas can make it dangerous for ships to enter them too deeply, but nebulas contain gasses that can be harnessed by ram scoops of ships and utilized as fuel or for other uses. The materials that make up some nebulas disrupt ship sensors and scanners, and some can even disrupt energy shields or weapons. [Star Trek 2]
Real Space is the dimension in which we live... the understood reality that is shown in science fiction without before entering other dimensions such as hyper-space, sub-space, or the time vortex.
Outside of planetary atmospheres, real space (normally called just space or sometimes outer space) is a near vacuum so that nothing can survive without some sort of adaptation for the extreme cold and pressures (or absence of pressures). Space contains, basically, everything.
Of important note is that space is apparently limitless. It is possible that space is... shperical and loops back around on itself.
Stars are large balls of burning gas/plasma, that generate great amounts of heat, light and gravity. Many stars have planets, comets, and the like in orbit around them. It is also possible for stars to orbit each other.
Stars also have a definite life span, where they condense, then grow into Red Giants (really huge, even for a star) and either burn out or fall in upon themselves to form a black hole.
Some theorize that stars can be used as gateways to wormholes.
Pulsars are stars that have a measurable rhythmic energy discharge which many star travelers use as navigational markers.
Sub-Space is a smaller space/dimension just "below" real space. It is frequently used as a medium for sending real-time messages over great distances. There are technologies that allow ships to reduce their inertia in real space by wrapping a piece of Sub-Space around themselves. Some other even more advanced technologies have gone so far as to use subspace as a form of cloaking device, where they achieve the effect of submerging their ships within sub-space. Generally, it is not possible for anything other than energy to travel in sub-space.
Time flows throughout the universe (multiverse), at the "center" of Time is the Time Vortex, which can be traversed by time travelers to to move from one moment into another... or even from one physical point to another, in addition to traveling between chronological points.
The Universe is everything. Galaxies, stars, time, physics... it may even be alive.
Voids are considered the single greatest danger to space travelers. A void is similar to a nebula in that it seems to be at least partly gaseous, but while a nebula can get very dense, that is not normally a problem with a Void. Voids are vast fields of nothingness... more so than space because they black out the stars, and act as magnetic storms to totally eliminate any possible methods of navigation including light from stars. Ships that enter voids can easily loose their way since they have no way to navigate... they can be lost forever, their pilots not realizing that they are flying in circles.
There are legends in just about every series about a void that is endless.
[The best examples of voids can be found in Battlestar Galactica, and ST:Voyager.]
Worm Holes are passages that cut through the fabric of space. Traveling by worm hole would cut travel time drastically. Natural worm holes are very unstable, with either end shifting randomly throughout space.
It does not seem very difficult to form worm holes... They can be made accidentally by FTL drive malfunctions, but controlling a Worm Hole is extremely difficult.
[Seen in Farscape, Star Trek:TMP, and other Trek shows.]