What Do We Know About Space?
How much do we know about space? Actually, not as much as we'd like to. Every time you open a stargazing app, you're probably amazed at how many different objects you can find there. But what if we told you that all those planets, stars, and other visible objects represent less than 5% of the Universe’s total content?
Contents
- What is space?
- What is space made of?
- What is in space?
- Brief summary: basic space facts to remember
Space is vast and full of mysteries. Many of them are yet to be solved, even though science has moved really far. Let’s sum up what is known at this point.
What is space?
Space is a near-perfect vacuum without any air. It is not empty: it contains many forms of radiation, as well as particles of gas, dust, and other matter floating around the void.
From Earth, we can observe planets, stars, and galaxies that are within 46.5 billion light-years in any direction. This region of space is called the observable Universe. The best current estimate of the Universe’s age is about 13.8 billion years.
What is outer space?
From our Earth-bound perspective, outer space is everything that lies outside the boundary separating the Earth from space. There are different definitions of where exactly outer space begins. The most widely used boundary is Kármán line, which sits about 100 km above mean sea level, though some sources place it slightly lower, at 80 km. Starting from this mark, the air becomes too thin for regular aircraft (relying on lift) to fly.
Outer space structure
Outer space can be divided into several regions. They are determined by magnetic fields and “winds” that dominate within them.
- Geospace is the region of outer space near the Earth. It lies between the Earth’s upper atmosphere and the outermost reaches of the Earth’s magnetic field.
- Interplanetary space is the outer space within the Solar System. It is defined by the solar wind, which forms a heliosphere – a giant “bubble” around the Sun and its planets. At heliopause (the outer edge of the heliosphere), it passes over into interstellar space.
- Interstellar space is the physical space between the star systems in a galaxy. It is filled with the interstellar medium (ISM), which consists of gas and dust.
- Intergalactic space is the physical space between galaxies. It is very close to a total vacuum, as it’s almost empty, but filled with extremely thin ionized gas.
What is space made of?
Scientists think the Universe consists of three types of substances: normal matter, dark matter, and dark energy.
Normal matter
Normal matter, also called ordinary or baryonic, consists of protons, neutrons, and electrons that make up every visible object around. Everything that we can see – stars, planets, trees, animals, and human beings – is made of normal matter. The proportion of normal matter in the Universe is surprisingly small – less than 5%.
Dark matter
Dark matter neither emits nor absorbs light and energy and is therefore completely invisible. Scientists can’t see it directly, but they infer its presence from its gravitational pull on normal matter. For example, spiral galaxies rotate so fast that the gravity from the visible matter alone wouldn’t be strong enough to hold them together – without dark matter, they would simply fly apart. In total, dark matter is thought to make up about 27% of the Universe. Possible candidates include:
- WIMPs (Weakly Interacting Massive Particles): Hypothetical particles that are much heavier than the particles making up ordinary matter. They interact only through gravity and the weak nuclear force, meaning they would pass through matter almost without a trace. Because of their mass, WIMPs could provide the extra gravity needed to explain the motion of galaxies.
- Axions: Theoretical particles that are extremely light and very weakly interacting. They were first proposed to solve a mathematical problem in particle physics, but later scientists realized they could also be a good candidate for dark matter.
- Sterile neutrinos: Neutrinos are particles that we know exist — tiny, almost massless, and rarely interacting with matter. Sterile neutrinos are a proposed heavier version of neutrinos: they do not interact through any of the known forces except gravity. Their extra mass and invisibility make them another possible explanation for the hidden matter in the Universe.
Dark energy
Dark energy can be thought of as a property of space itself, causing the accelerated expansion of the Universe due to its negative pressure, rather than a force directly repelling objects. The concept of dark energy has been proposed by scientists to explain why the Universe is not just expanding but is doing so at an increasing rate. For now, scientists have not defined its nature and origin: here, “dark” rather means “unknown” than literally dark, as is the case with dark matter. Dark energy is estimated to account for roughly 68% of the Universe.
What is in space?
Space isn’t empty — it’s packed with objects of every size, from tiny rocks to immense walls of galaxies. Let’s start with the smaller objects — planets, moons, and comets — before moving outward to stars, galaxies, and the largest cosmic structures.
Planets, moons, and small bodies
Our exploration of the Universe begins close to home — with the Solar System, our immediate cosmic neighborhood. Here we find planets, moons, comets, and an entire belt of asteroids between Mars and Jupiter. Astronomers have also spotted exoplanets orbiting distant stars and even interstellar comets passing through from other star systems.
Planets
A planet is an object that orbits a star, is massive enough for its own gravity to pull it into a nearly spherical shape, and has cleared its orbit of other debris. In the Solar System, there are two main types of planets:
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The rocky worlds (terrestrial planets) — Mercury, Venus, Earth, and Mars. These small and dense planets are composed mainly of rock and metals such as iron, magnesium, and silicon. Mercury, despite being a planet, has a thin “tail” of sodium atoms swept away by the solar wind, giving it a comet-like appearance. Venus is also known to have an ion tail.
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The giant planets — Jupiter and Saturn are true gas giants, composed primarily of hydrogen and helium, while Uranus and Neptune are classified as ice giants, with large amounts of water, ammonia, and methane in addition to hydrogen and helium.
Have you ever wondered which Solar System planet matches your vibe the most? Take our quiz and discover your spirit planet!
We also have dwarf planets — smaller worlds that orbit the Sun but, unlike the “full-fledged” planets, haven’t cleared their orbits of debris. Because of this, they’re usually left out of the main “pantheon” of Solar System planets. The most famous is Pluto, but there are several others, such as Eris, Haumea, Makemake, and Ceres.
Beyond our Solar System, astronomers have discovered thousands of exoplanets – worlds orbiting other stars. Some of them push the boundaries of imagination. Astronomers have detected planets with unusual compositions — including one thought to be rich in carbon, possibly forming a world with vast amounts of diamond-like material. Learn more about this and many other unusual space objects in our article.
Moons
A moon is an object that orbits a planet or something else that is not a star (e.g. an asteroid). Our planet Earth has just one moon, while some planets have dozens of them, and others don’t have any at all.
- As of 2025, Saturn holds the record with 274 confirmed moons, just ahead of Jupiter with 95. The lead has shifted back and forth in recent years as astronomers keep finding new small moons around both giants.
- If we rank all the moons of the Solar System by size, three of the top five — Ganymede, Callisto, and Io — orbit Jupiter. The other two are Titan (Saturn’s largest moon) and our own Moon, which proudly takes fifth place on the list!
Asteroids
Asteroids are the rocky remnants of the Solar System’s formation about 4.6 billion years ago. Most orbit in the asteroid belt between Mars and Jupiter, ranging in size from tiny boulders to dwarf planets like Ceres.
- Some asteroids occasionally come close to Earth, like Apophis, which will pass within 32,000 km (10 times closer than the Moon) of our planet in 2029. It will even be visible in the sky as a naked-eye object! Explore our article about Apophis to get more details.
Comets
Comets are the icy bodies that travel on highly elongated orbits. Many comets come from the distant Oort Cloud, others from the Kuiper Belt, and a few — like 2I/Borisov and 3I/ATLAS — from interstellar space. Why do comets have such spectacular tails, and what do they tell us about the Solar System? Find out in our article about comets.
Stars and nebulae
A star is a giant ball of hot, glowing gas — mostly hydrogen and helium — held together by gravity. In its core, atoms fuse in nuclear reactions, releasing enormous amounts of energy that make the star shine. Stars are born inside nebulae, immense clouds of gas and dust where gravity pulls material together until nuclear fusion ignites.
- One of the most unusual nebulae is the Boomerang Nebula, about 5,000 light-years away in Centaurus. Its interior is the coldest place ever found in the Universe, with a temperature of –272 °C, just one degree above absolute zero.
After forming, stars shine for billions of years before ending their lives in different ways. Some become white dwarfs — faint, dense stellar remnants. A famous example is the bright star Sirius, which is actually a binary system: the hot, luminous main star, Sirius A, and a faint white dwarf, Sirius B. More massive stars may collapse into neutron stars, only a few kilometers across, or even into black holes, whose gravity is so strong that not even light can escape.
- Among the most ancient stars still shining is HD 140283, a subgiant in Libra nicknamed the Methuselah — after the biblical figure said to have lived 969 years. It is estimated to be 12–14 billion years old, nearly as old as the Universe itself.
To better understand how stars evolve — from birth in a nebula to their spectacular endings — check out our infographic on the life cycle of a star.
Galaxies and quasars
Galaxies are immense systems of billions of stars, bound together by gravity along with gas, dust, and dark matter.
- Our own Solar System is located in the Milky Way galaxy, whose bright band of stars can be seen stretching across the night sky.
- The nearest large neighbor, the Andromeda Galaxy, is so big and bright that it can be spotted with the naked eye under dark skies.
Like stars, galaxies also have their life cycle. Quasars are thought to represent active galactic nuclei at an early stage of galactic evolution, when a central supermassive black hole consumes surrounding matter at a tremendous rate. This process releases extraordinary amounts of energy, making quasars some of the most luminous objects in the Universe.
- Quasar 3C 273 in Virgo was the first quasar ever discovered and remains the brightest as seen from Earth (apparent magnitude 12.9). Radio observations reveal brightness temperatures up to 10¹³ K (10 trillion °C) — a measure of radiation intensity, not the actual plasma temperature.
- Quasar APM 08279+5255, located 12 billion light-years away in Lynx, is surrounded by a giant cloud of water vapor containing about 140 trillion times the mass of all Earth’s oceans.
Large-scale structures
Galaxies do not float in isolation. They cluster together into groups like the Local Group of Galaxies and the Virgo Cluster, which themselves belong to even larger structures such as the Laniakea Supercluster.
- The Hercules–Corona Borealis Great Wall, or simply the Great Wall, is a candidate for the largest structure in the observable Universe, though its status is still debated. It could be as large as 15 billion light-years across and possibly contains billions of galaxies. Located roughly 10 billion light-years away, it lies in the direction of the Hercules and Corona Borealis constellations.
F.A.Q.
How old is space?
The best current estimate of the Universe’s age is about 13.8 billion years. To help you visualize the history of the Universe, we compressed it into 1 Earth year and got a cosmic calendar. Check it out in our infographic.
Where does space begin?
Space does not begin at a sharply defined altitude above Earth’s surface. A widely accepted boundary is the Kármán line, set at 100 km (62 miles) by the FAI (Fédération Aéronautique Internationale). Some organizations, such as NASA and the U.S. Air Force, instead use a slightly lower limit of 80 km (50 miles). These heights are chosen because above them the atmosphere is too thin to generate enough lift for aircraft wings to function, meaning that aerodynamic flight is no longer possible – only orbital or rocket propulsion works.
How big is outer space?
The observable Universe – the part we can visibly see and measure – is estimated to be about 46.5 billion light-years in any direction from the Earth. If we imagine it as a sphere surrounding our planet, its diameter will be around 93 billion light-years. Find our location in the observable Universe using our infographic.
What is the temperature of space?
The baseline temperature of outer space is set by the cosmic microwave background (CMB) radiation, the afterglow of the Big Bang. It corresponds to about 2.7 K (−270 °C).
What color is space?
We know from experience that the space appears black. However, taking into account that the Universe is infinite and contains billions of stars, shouldn’t it be bright white? This oddity is known as Olber’s paradox; see its possible solutions in our dedicated article.
Why can’t sound travel through space?
Sound is a mechanical wave that requires a medium, such as air or water, to propagate. Space is a vacuum: there is no air, and sound has no way to travel. That’s why space is generally considered to be silent.
Is space completely silent?
Though space is a vacuum, it’s not completely empty: it is filled with plasma, or charged particles. These particles can generate (or be affected by) electric and magnetic fields, and, thus, can transfer magnetosonic waves – the plasma-equivalent of sound waves. They are inaudible to humans, but can be recorded by spacecraft and converted into audible tracks – some weird-sounding “space music”.
Brief summary: basic space facts to remember
Space is a vacuum with radiation and matter particles floating out there. The estimated age of the Universe is about 13.8 billion years. The size of the observable Universe is about 46.5 billion light-years in any direction from the Earth (or 93 billion light-years in diameter). Everything that exists in the Universe is made of normal matter, dark matter, and dark energy; scientists are still investigating the nature and origin of the latter two substances.
All about space: get even more space information
If you enjoyed this article and are looking to learn more about space and the Universe, check out this related content:
- Learn more about space junk and the way it affects humanity;
- Find out the difference between light-year, astronomical unit, and lunar distance with our infographic;
- Recall everything you know about space and take a quiz: tell the true space facts from the fictional ones!
