Space is big. The Voyager spacecraft (Voyager I and II) were launched in 1977 and are, forty years later, only just entering interstellar space. Though the exact point at which space becomes “interstellar” is debatable.
The Voyagers will take 40,000 years or so to reach one of the stars in the “local” group. That’s about one fifth of the time that humans have existed as a separate species. Or 400 times as long as the length of time that a human is able to live. If a generation is around 20 years long, that is about 2,000 generations. It is a long, long time, and we may well be extinct as a species by then, for one reason or another.
The “local group” of stars is an arbitrary group of stars which are (relatively) close to the Sun. I’m unsure whether they really constitute a group of bodies bound by gravity or whether they are close to the sun by chance. Of course if any of the stars in the local group are bound by gravity, then the stars would form a binary or multiple star system.
Of course, our star and all the others in the local group are part of our galaxy, the Milky Way. Specifically we are part of one of the arms of the Milky Way, which is a spiral galaxy. All stars in the Milky Way are bound by gravity, with the possible exception of stars which are merely passing through the Milky Way at this time.
Just like stars, galaxies seem to form groups, which then form super-groups and so on. All these structures are several orders of magnitude larger than the prior smaller ones, are more complex and contain more matter.
The majority of space however is just space. The gaps between the bits of matter, stars, systems, galaxies, groups and so on contain almost nothing, or a seething sea of virtual particles depending on how you look at it.
The “almost nothing” consists of a very small number of particles (usually hydrogen atoms or nuclei, protons) in a cubic metre. For comparison the best vacuum that can be created on Earth may contain several million atoms in that volume. This of the same order of magnitude as molecular clouds as observed by astronomers. Molecular clouds are among the densest clouds observed in space.
The stars, planets, asteroids and similar bodies comprise only a very small part of the Universe and the average density of the Universe is much the same as the density of empty space. In other words, the Voyagers are heading into areas where the conditions are more typical of the Universe than those around our star.
I mentioned virtual particles earlier. While virtual particles show up as short-lived particles that briefly come into existence in some particle interactions, the virtual particles that I refer to come into existence in a vacuum as pairs and almost immediately mutually annihilate. Though they do not interact with other matter, they do have an effect which can be measured.
Mathematicians have a different concept of space. In mathematics space is a (usually) three dimensional construct that serves merely to separate and give structure to such things as points, lines, planes, volumes and shapes. Point A is distinguished from point B by the distance between them and also the orientation of a line joining them.
In a simple case every point has (usually) three coordinates which define its position relative to some fixed point or origin and fixed coordinate system. The coordinate system can be any system that locates the point.
For instance, you can describe the point A’s position as “Face along a given axis, rise up until you are level with point A. The distance moved up is one coordinate. Rotate left through an angle until a line parallel to the plane you rose up from passes from you through the point A. The angle you turned through is the second coordinate. The third coordinate is the distance along the line from you to the point.
I’ve described a cylindrical coordinate system, but the coordinate system may be any system that gives three unique (in that system) coordinates for point A. A common system is the Cartesian system of three mutual perpendicular axes. Another is the spherical system, defined by two angles and a distance.
Of course, such systems can be generalised to more dimensions or fewer, depending on the needs of the mathematician. Most people can understand simple two dimensional graphs which are usually drawn using a two dimensional Cartesian coordinate system.
Of course scientists use mathematical models for various purposes. For instance the scientist may wish to know the probability of one hydrogen atom in the interstellar space meeting another such molecule. Since we have only about one such atom in every cubic metre, the probability is going to be small, but, of course, we can assume as lone a time period as we wish.
Gravity has to be figured in, to be sure, but a long time will be required for such atoms to collide. If the atoms are by chance moving slowly relative to each other, they may stick together and form the basis of a particle of matter. Such a clump might attract other atoms and before long (well actually after literally an astronomical length of time) a star will form.
It must have happened otherwise we would not be here. We are the result of matter aggregating and then exploding. All the atoms in our bodies that are not hydrogen were made in the centres of stars. The stars have to have exploded to allow these atoms to end up in our bodies.
A long time has passed since the birth of the Universe. In that time matter has crept together hydrogen atom by hydrogen atom until great collections of atoms have compressed in the centre to the point where nuclear reactions have occurred. Hydrogen fused to helium, then to heavier elements all the way up to Uranium.
At which point the stars have exploded throwing all the elements out into the Universe. These elements then crept together again to produce new stars like our sun and gaseous and rocky planets orbiting them. Prior to this there were no rocky planets and no life. We live in the Universe Mark II.