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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThe most surprising fact about space is that you are already in it. Earth is a planet moving through the universe, so the phrase “out in space” describes a place you have been all along. From there, the scale grows quickly: the solar system stretches far beyond the planets, the Milky Way holds at least 100 billion stars, and the observable universe contains at least 100 billion galaxies. Along the way, some of the strangest objects in physics, black holes, can be studied even though light cannot escape them.
You are technically in space right now
NASA’s own wording is blunt: “In fact, you’re technically in space right now.” The everyday idea of space as a distant place beyond the sky is a matter of convention. Earth’s atmosphere thins gradually, and there is no sharp line where “air” ends and “space” begins. Engineers and international bodies use a convention near 100 kilometers (about 62 miles) altitude, known as the Kármán line, but that boundary is a human definition, not a physical wall.
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NASA also describes the universe as nearly 14 billion years old. The phrase “nearly” is deliberate: it is a rounded cosmological estimate, and the exact figure carries an uncertainty that astronomers continue to refine.
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Three scales are often blurred together. They are worth keeping separate because each uses a different measuring stick.
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| Scale | What it contains | How the number is expressed |
|---|---|---|
| Solar system | The Sun, eight recognized planets, and their moons, asteroids, and comets | Distances in astronomical units; 1 astronomical unit is about 150 million kilometers (93 million miles), the average Earth–Sun distance |
| Milky Way | At least 100 billion stars, according to NASA Science | A lower bound on the star count, not a precise census |
| Observable universe | At least 100 billion galaxies, according to NASA Science | A lower bound; the observable universe is the part whose light has had time to reach us |
NASA Science notes that if every galaxy were the same size, the arithmetic would suggest roughly 10 sextillion stars in the observable universe. That figure is an illustrative calculation built on an assumption that does not hold in reality, so it should not be read as a count of stars.
What the universe is made of
Most of the universe is not the matter we can see. NASA Science gives the breakdown as follows:
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- Stars, planets, and other ordinary matter: less than 5 percent
- Dark matter: about 27 percent
- Dark energy: about 68 percent
These percentages come from cosmological models and measurements of how the universe behaves on large scales. They are estimates, not a direct inventory of visible objects, and dark matter and dark energy have not been observed directly as particles or fields. The gap between what we can see and what the models require is one of the open problems in modern cosmology.
The solar system reaches far beyond the planets
The planets orbit inside a much larger region shaped by the Sun. The solar wind, a continuous stream of charged particles from the Sun, pushes outward and inflates a bubble called the heliosphere that extends well past the orbit of Neptune. NASA’s solar system lithograph, published by NASA/JPL in 2013, reports that the Voyager spacecraft have crossed the termination shock, the boundary where the solar wind slows down as it meets the surrounding interstellar medium.
Moon counts are a useful example of a number that changes. The same 2013 NASA/JPL lithograph reports 146 known planetary moons, with at least 27 more awaiting official recognition. That is a historical count from 2013. Newer discoveries have been announced since then, so treat the figure as a snapshot rather than a current total.
Black holes: what they are and how we know they exist
A black hole is not an empty hole in space, and it does not have a visible solid surface. NASA Space Place describes it as a region where gravity is so strong that nothing, not even light, can escape. The boundary associated with that escape limit is the event horizon. Because light cannot leave, black holes cannot be seen directly. Astronomers detect them through their effects on nearby matter and through gravitational waves.
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Two effects matter most. Gas falling toward a black hole heats up and can emit radio waves and X-rays, which telescopes can detect. Pairs of black holes that spiral together and merge send out ripples in spacetime called gravitational waves. NASA’s black hole overview describes these ripples as predicted by Einstein’s general theory of relativity.
NASA and ESA both describe two broad classes of black hole, which differ greatly in mass:
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| Type | Approximate mass range (per NASA) | How it is understood to form | How it is observed |
|---|---|---|---|
| Stellar-mass black hole | A few to dozens of solar masses | Collapse of the core of a sufficiently massive star after it exhausts its fuel | Hot matter emitting X-rays and radio waves; gravitational waves from mergers |
| Supermassive black hole | Hundreds of thousands to billions of solar masses | Found at the centers of most large galaxies; how the largest ones formed remains an open question, according to ESA | Motion of nearby stars and gas; imaging of surrounding matter |
The size of supermassive black holes is best illustrated with specific measurements. NASA’s 2023 scale article gives Sagittarius A*, the black hole at the center of the Milky Way, as about 4.3 million solar masses, based on long-term tracking of stars orbiting it. The same article reports TON 618 at more than 60 billion solar masses. That value is NASA’s reported measurement, and it should not be presented as a definitive ranking of the largest known black hole, since mass estimates for distant objects are revised as observations improve.
NASA Goddard astrophysicist Ira Thorpe has described the detection method this way: “Since 2015, gravitational wave observatories on Earth have detected the mergers of black holes with a few dozen solar masses thanks to the tiny ripples in space-time these events produce.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Milestones in observing black holes
- 2015: LIGO first detected gravitational waves from a black-hole merger, confirming a prediction made about a century earlier.
- 2019: The Event Horizon Telescope, an international collaboration linking eight ground-based radio telescopes, released the first image of a black hole. It shows the galaxy M87’s central supermassive black hole as a dark region silhouetted against glowing surrounding matter. It is not a photograph of light escaping from inside the event horizon, because no such light exists to capture.
Planets around other stars
Earth is not the only planet orbiting another star. NASA Science says astronomers have discovered thousands of planets around other stars, a discovery enabled by increasingly sensitive telescopes. The count keeps rising, so check current NASA exoplanet pages for the latest total.
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Look up on a clear night
Many of the facts above can be seen with simple tools. On a dark, clear night, the Milky Way appears as a faint band of stars across the sky, and binoculars reveal far more of its stars than the unaided eye. A telescope can show the Moon’s craters and some of the bright planets in detail. None of these views match the images produced by professional observatories, which combine large mirrors, long exposures, and computer processing. Their value is in seeing the scale for yourself.
For the official source material behind this article, start with NASA’s explainer on the universe at science.nasa.gov/exoplanets/what-is-the-universe/ and NASA’s black hole overview at nasa.gov/universe/what-are-black-holes/.
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