A note on the numbers
The facts below are widely accepted and rounded. Many cosmic quantities are estimates that scientists refine over time, so where a figure is uncertain it is given as “about” or “roughly” and not as an exact value.
1. The universe is about 13.8 billion years old
This age comes from measuring how fast space is expanding and what the universe contains, and it agrees with other methods, such as the ages of the oldest stars. The Big Bang model describes how the universe has evolved from a hot, dense early state; it does not describe an explosion at a place in space. Earth is much younger, at roughly 4.5 billion years.
2. We only see part of it
The observable universe is roughly 93 billion light years across. That is not the whole universe, only the region from which light has had time to reach us. Nobody knows how far the rest extends.
3. There are hundreds of billions of galaxies, at least
Estimates for the number of galaxies in the observable universe range from hundreds of billions upward, and the true figure depends on how many faint, small galaxies telescopes can detect. Our own Milky Way holds on the order of a hundred billion or more stars.
4. Looking far away means looking into the past
Light travels at a finite speed, about 300,000 kilometers per second. Sunlight reaches us in a little over eight minutes, moonlight in about a second, and light from the nearest big galaxy, Andromeda, in roughly 2.5 million years. Telescopes therefore show distant objects as they were long ago. It is the same reason a light year is a measure of distance, not time.
5. Most of the universe is not the stuff we know
Current measurements suggest that ordinary matter, the atoms in stars, planets and people, makes up only about 5 percent of the universe’s total content. Roughly 27 percent is attributed to dark matter, detected through its gravitational effects but not yet identified, and about 68 percent to dark energy, linked to the accelerating expansion of space. These percentages are rounded and come from a standard model that could be revised.
6. Ordinary matter is mostly hydrogen and helium
By mass, ordinary matter is around three quarters hydrogen and a quarter helium, with only a small fraction of everything heavier. Hydrogen and most helium date from the early universe.
7. You are partly made of stars
Elements such as carbon, oxygen and iron were produced inside stars and in stellar explosions. When stars reach the end of their lives, they spread these elements into space, where they can become part of new stars, planets and living things. Calling people “stardust” is a poetic description of a literal fact. The story of how it happens is part of the stellar life cycle.
8. Space is mostly empty
Galaxies are separated by huge distances, and even inside a galaxy, the stars are far apart compared with their sizes. If the Sun were shrunk to the size of a grapefruit, the nearest other star would be thousands of kilometers away. That kind of comparison depends on the chosen sizes, but the point holds: the average density of space is extremely low.
9. The Sun holds nearly all the solar system’s mass
The Sun contains about 99.8 percent of the mass of the solar system. Jupiter, the largest planet, accounts for most of what remains, and it is more massive than all the other planets put together.
10. A day on Venus is longer than its year
Venus rotates very slowly, taking about 243 Earth days to turn once, while it goes around the Sun in about 225 Earth days. Its rotation is also backward compared with most planets.
11. The Milky Way and Andromeda are expected to merge
The two galaxies are approaching one another and are expected to merge in a few billion years. Given the large spaces between stars, individual star collisions would be rare. The merged galaxy would likely be a different shape from either one.
12. The sky is full of other worlds
Astronomers have confirmed thousands of planets orbiting other stars, and statistics suggest that planets are common. Whether any of them have life is unknown, and the question of alien life remains open.
What these facts have in common
Many of the most striking facts in cosmology come from only a few ideas: light takes time to travel, space can expand, and gravity gathers matter into structures. When a number sounds extreme, it is worth checking whether it is a measurement, an estimate or a model-dependent calculation. Reliable sources usually state which.