"The cosmos is within us throughout us; we are made from its primordial atoms and energies, both enriched and tamed by generations of long-vanished stars. When the Sun dies, in four or five billion years, the same elements of life will be returned to the cosmos, further enriched. After much drifting and mixing, the process will gradually begin again, elsewhere in the Milky Way, itself one galaxy of billions of stars among billions of similar galaxies throughout the Universe. While we and our planet are rare concentrations of matter, the universe itself is mostly empty, and on average contains a few dozen atoms in a room-sized volume. Only within dusty nebulae within the gavitational embrace of a galaxy can life form --- probably.
The Universe is big, very big. Astronomers measure its vastness by how far light travels in one year. Light travels quickly --- 300,000 kilometres a second --- a billion kilometres per hour --- around the Earth seven times a second; in a year it traverses ten trillion kilometres (six trillion miles) and travels from the Sun to Earth in about eight minutes. It travels from the edge of the boundless Universe in 13.7 billion years. The Universe is boundless because it has no centre and no edge: its dimensions are defined by the travel time of light since the Big Bang spirited it into existence, 13.7 billion years ago, a fact that links time and space together into the four-dimensional fabric of spacetime.
Most of the 'stuff' of the Universe is in the form of Dark Matter and Dark Energy, the nature of which remains to be unravelled. The only measurable property of Dark Matter is its mass; it is dark because it does not absorb, reflect or emit any kind of radiation. It remains unseen, and its presence is inferred from its gravitational effects on galaxies and on light, which is bent by gravity. Dark Energy is an even more mysterious force, which is accelerating the expansion of the universe as would negative gravity. Since mass and energy are interchangable we are startled to find that invisible Dark Matter and repulsive Dark Energy together comprise about 95 percent of the Universe. The rest, you and me, the stars and galaxies, our experiences and our lives exist in the remaining five percent.
Of that tiny fraction we can see, the most compelling pictures from space are of the nebulae --- cosmic clouds of dust and gas --- many of which measure many light years across. A few dust particles are a tenth of a millimetre in size, most are a thousand times smaller, more like smoke than the dust of the desert. Its density can reach one particle per cubic centimetre ... it does near the sun, but elsewhere it can be much lower (1 particle/cubic km) or 1000 x higher in a molecular cloud ..."