What Do We Know About Space?

Star Walk
6 min readMay 8, 2023


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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 the Earth, we can observe planets, stars, and galaxies that are within 46.5 billion light-years in any direction from our planet. This region of space is called the observable universe. The estimated age of the universe is from 11.4 billion to 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 Karman’s line, which sits 100 km above mean sea level. Starting from this mark, the air becomes too thin for regular aircraft (relying on lift) to fly.

Space regions

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 generally free of dust and debris.

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, thus, is completely invisible. Scientists assume it is composed of non-baryonic matter, such as WIMPs (weakly interacting massive particles), neutralino, and neutrino. Though dark matter can’t be observed directly, astronomers assume it exists because of its gravitational effect on normal matter. For instance, the examination of spiral galaxies has shown that they contain more mass than could be seen. Without dark matter, the galaxies would just fly apart, for the gravitational force of the normal matter alone is not enough to pull together all the particles. Dark matter supposedly makes up about 27% of the universe.

Dark energy

Dark energy is a hypothetical form of energy that repels gravity: it pulls cosmic objects apart while gravity draws them close together. 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 can you find in outer space: top 5 exotic objects

The deeper the scientists dive into the cosmic depth, the more unusual objects and locations they discover. Here are some of the most outstanding record-holders in space.

The coldest object in space


The coldest place found in the universe so far is the Boomerang Nebula. It’s located some 5,000 light-years away from the Earth in the constellation Centaurus. The temperature of its deep interior is -272 °C — one degree away from absolute zero.

The hottest place in space


Quasar 3C 273 in the constellation Virgo is one of the hottest regions in space. The temperature here varies from 20 to 40 trillion °C. 3C 273 is also the first quasar to be ever found and the brightest quasar as seen from the Earth (its apparent magnitude is 12.9).

The oldest star in space

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Methuselah (HD 140283, HIP 76976) is 16 billion years old, which makes it the oldest star in space — somehow, even older than the universe (scientists are still figuring out how this is possible). The star is located in the constellation Libra and can be observed with binoculars (its apparent magnitude is 7.2).

The biggest object in space

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The Hercules–Corona Borealis Great Wall, or simply the Great Wall, is the largest known structure in the observable universe. It is 10 billion light-years across and could contain billions of galaxies. It lies about 10 billion light-years away in the direction of the Hercules and Corona Borealis constellations.

The biggest water source in space

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The largest reservoir of water in space holds 140 trillion times more water than all the oceans on our planet. It is a cloud of water vapor surrounding a quasar APM 08279+5255 that is located in the constellation Lynx, 12 billion light-years away from us.

The list of the most bizarre objects in space can be infinite: it can also include a planet made of diamonds, the only Solar System planet that has a tail, and the celestial body where a man is buried. Learn more about these space objects in our article.


How old is space?

There are two competing estimates based on the two different measurements, suggesting the universe could be from 11.4 billion to 13.8 billion years old. 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 doesn’t begin at a definite altitude above the Earth’s surface. There is a conventionally accepted border called Karman’s line, which sits 100 km (62 miles) above mean sea level.

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 -270 °C. However, there are some extreme spots: the temperature in the coldest place in space is -272 °C; in the hottest place, it varies from 20 to 40 trillion °C.

What color is space?

We know from experience that the space is 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](https://starwalk.space/news/why-is-the-sky-dark-at-night).

Why can’t sound travel through space?

The sound is vibrating air. The space is a vacuum: there is no air out there. Thus, sound has no way to travel.

Bottom line

Space is a vacuum with radiation and matter particles floating out there. The estimated age of the universe is from 11.4 billion to 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.



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