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Earth & Space Science · Foundations

Universe

9 min read 5 practice questions
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

The is everything that exists: all of space, time, matter, and energy. It is about 13.8 billion years old, and the part we can observe contains billions of galaxies. Those galaxies are not scattered at random. Gravity gathers them into groups, clusters, and superclusters that form a filamentary pattern called the . Nearly every distant galaxy is moving away from us, and the farther away it is, the faster it recedes — . Most of the universe's contents are and , known only through their effects.

Why this matters

Everything you have ever seen — Earth, the Sun, the stars — belongs to one connected system, and the universe is that system's full extent. Understanding its age, scale, and structure reframes basic questions about our place in it, from where the elements in our bodies came from to what will happen to everything in the distant future. The same observations that revealed cosmic expansion and dark matter also demonstrate how evidence can overturn intuition, a lesson that applies far beyond astronomy. Cosmology still drives telescope and computing technology, and its open questions, such as what dark energy is, are where the next generation of research begins.

The college version

Everything: matter, energy, and space

The universe is the entirety of what exists. NASA's glossary defines it as everything that exists, including the totality of time and space and all of the matter and energy within them — the entire cosmos. That includes every star and galaxy, the thin gas between them, and the space itself that stretches between one object and the next. It also includes the light, radiation, and gravitational influence that travel through that space. One useful distinction separates the universe as a whole from the : the sphere of space from which light has had time to reach us since the beginning. Telescopes cannot see beyond that boundary, and astronomers do not know how much universe lies beyond it — the observable part is simply all we can study. The observable universe alone holds billions of galaxies, some of them mapped up to 10 billion light-years from Earth.

About 13.8 billion years old, and expanding

The universe has a history. NASA dates its beginning to a rapid expansion some 13.8 billion years ago, and cosmologists describe the universe today as about 13.8 billion years old. How do we know it is expanding? In the 1920s, astronomers measuring the spectra of galaxies found that their light was shifted toward longer wavelengths, as if the galaxies were receding from us. Edwin Hubble combined those velocity measurements with distance estimates and, in 1929, published a striking relationship; with Milton Humason he confirmed in 1931 that a galaxy's recession velocity is directly proportional to its distance. This relationship, now called Hubble's law, is written v = H x d, where H is the Hubble constant. The more distant a galaxy is, the faster it moves away. The expansion is not a motion of galaxies through fixed space; the space between them is stretching, so expansion looks the same from every galaxy. What started the expansion — the big bang — is the subject of its own topic; here the point is the expansion itself.

The cosmic web: groups, clusters, superclusters, filaments

Galaxies are not spread uniformly through space; gravity organizes them. The Milky Way belongs to the Local Group, a small collection spread over roughly 3 million light-years with about 60 members, including the Andromeda galaxy. Much larger collections, called galaxy clusters, are bound together by gravity and can hold thousands or even tens of thousands of galaxies; the nearest rich example, the Virgo Cluster, lies about 50 million light-years away. Clusters and groups in turn gather into superclusters, structures hundreds of millions of light-years across. Survey maps such as the Sloan Digital Sky Survey show that clusters are strung along filaments and sheets that wrap around vast empty regions called voids, some 150 million light-years across. NASA calls the resulting pattern the cosmic web: galaxies, groups, clusters, and superclusters arranged in twisting, threadlike structures. The largest known walls of galaxies, such as the Sloan Great Wall, stretch about 1.4 billion light-years.

Dark matter and dark energy: the invisible 95 percent

Visible matter is only a small part of the universe's contents. In the current model, ordinary matter — everything made of atoms — makes up about 5% of the universe, dark matter about 27%, and dark energy about 68%. Dark matter is inferred from its gravity. In the 1970s, Vera Rubin found that stars at the outer edges of spiral galaxies orbit as fast as stars nearer the center, which requires far more mass than the visible stars and gas can supply. Rotation curves like these imply a large halo of unseen matter around each galaxy. Galaxy clusters show the same problem: their galaxies move too fast for the visible mass, and clusters act as gravitational lenses, bending light from background galaxies in ways that let astronomers map the hidden mass. Dark matter does not absorb, reflect, or emit light, and it has not yet been detected in laboratories. Dark energy is different: it is the name for whatever is causing the expansion to speed up. Supernova observations in the late 1990s showed that the expansion of the universe has been accelerating, and NASA states plainly that we do not yet know what dark energy is. Both are evidence-based parts of the current model, not settled facts about their identity.

How we know: light and telescopes

Everything in this lesson rests on light. Telescopes collect light from distant objects, and modern instruments gather it across the electromagnetic spectrum, from radio to gamma rays. Because light travels at a finite speed, looking farther into space means looking further back in time: the light from a galaxy 10 billion light-years away left that galaxy 10 billion years ago. Spectra of that light reveal what the galaxy is made of and how fast it is moving toward or away from us, which is how the recession velocities behind Hubble's law were measured. The same method — collect light, split it into a spectrum, measure brightness and position — supports the maps of the cosmic web and the inferences about dark matter. Telescopes are the universe's window, and every claim in this lesson is a claim about light that has crossed space to reach us.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

The universe is everything that exists — every star, galaxy, atom, and all of space itself — and it is about 13.8 billion years old. The part we can see holds billions of galaxies, and they are not scattered randomly. Gravity pulls galaxies into groups and clusters; clusters gather into superclusters; and on the largest scale everything is strung along filaments, forming a pattern scientists call the cosmic web. The universe is also expanding: nearly every distant galaxy is moving away from us, and the farther away it is, the faster it goes. That is Hubble's law. Most of the universe is stuff we cannot see: dark matter, which we detect by its gravity, and dark energy, which seems to be speeding the expansion up.

Picture it like this

Imagine a loaf of raisin bread rising in the oven. The raisins do not swim through the dough; the dough itself stretches, carrying every raisin farther from every other raisin. From any raisin, all the others appear to move away, and the more distant ones recede fastest — exactly what Hubble's law describes for galaxies.

Where the picture stops working

The bread has an edge and a center, while the universe, as far as we can tell, has neither — expansion looks the same from every galaxy. The analogy also ignores gravity, which holds galaxies, clusters, and superclusters together even as space stretches, which is why the Local Group is not being torn apart.

Worked example

Hubble's law can be turned around to estimate distances. Suppose a galaxy's spectrum shows it receding from us at 2,200 kilometers per second. Using v = H x d with H about 22 kilometers per second per million light-years, the distance is d = v / H = 2,200 / 22 = 100 million light-years. The galaxy moves away 22 kilometers per second faster for every additional million light-years of distance — the direct proportionality at the heart of the law. A galaxy receding at half that speed would be about 50 million light-years away. Applied to thousands of galaxies, the same arithmetic underlies the three-dimensional maps that reveal the cosmic web.

Key takeaway

The universe is everything that exists — about 13.8 billion years old, expanding according to Hubble's law, arranged into the cosmic web, and dominated by dark matter and dark energy whose true natures are still unknown.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

According to NASA's glossary, what is the universe?

Choose an answer, then check it.
Question 2 of 3intermediate

What does Hubble's law state about distant galaxies?

Choose an answer, then check it.
Question 3 of 3intermediate

Astronomers measure a galaxy receding from us at 2,200 kilometers per second. Using Hubble's law with H about 22 kilometers per second per million light-years, about how far away is the galaxy?

Choose an answer, then check it.
Practice all 5

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Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Define the universe as everything that exists — all matter, energy, space, and time — and distinguish it from the observable universe.
  • Explain Hubble's law and what it shows about the expansion of the universe.
  • Describe the large-scale structure of the universe: galaxy groups, clusters, superclusters, filaments, and the cosmic web.
  • Explain the evidence for dark matter and dark energy and why they are part of the current model of the universe.
  • Apply the idea that astronomers learn about the universe from light gathered by telescopes.

Common mistakes

  • The expansion means galaxies are flying through space away from a central point, with the Milky Way at the middle.

    Hubble's law describes space itself stretching, so every galaxy sees other galaxies receding in the same way; there is no center, and the Milky Way is not special.

  • The universe must be 13.8 billion light-years across because it is 13.8 billion years old.

    13.8 billion years is an age, not a size. The observable universe is the sphere whose light has reached us, and the universe as a whole may extend far beyond what we can see.

  • Dark matter is ordinary matter that is simply too faint or too far away to see.

    Dark matter does not absorb, reflect, or emit light at all, and it has not been detected in laboratories; it is known only through gravity, such as rotation curves and gravitational lensing.

  • Dark energy is a force pushing galaxies through space.

    Dark energy is the name for whatever is accelerating the stretching of space itself; its nature is unknown, and NASA's current description treats it as a mystery, not a known force.

Easily confused

Observable universe vs. Universe as a whole

The observable universe is the sphere of space from which light has reached us — all we can study; the universe as a whole is everything that exists, and its full extent is unknown.

Dark matter vs. Dark energy

Dark matter is inferred from gravity that holds galaxies and clusters together (about 27% of the universe); dark energy is inferred from the accelerating expansion (about 68%), and neither has been directly detected.

Galaxy cluster vs. Supercluster

A cluster is a group of galaxies bound together by gravity; a supercluster is a much larger gathering of clusters, groups, and galaxies that are typically not gravitationally bound to one another.

Key vocabulary

Universe
Everything that exists, including the totality of time and space and all of the matter and energy within them; the entire cosmos.
Observable universe
The region of the universe from which light has had time to reach us since the beginning; the part telescopes can see.
Galaxy cluster
A collection of galaxies bound together by gravity, ranging from dozens of members to rich clusters with thousands of galaxies.
Supercluster
A large gathering of galaxy clusters, groups, and individual galaxies spanning hundreds of millions of light-years.
Cosmic web
The large-scale pattern of the universe, in which galaxies, groups, clusters, and superclusters lie along twisting, threadlike filaments separated by vast voids.
Hubble's law
The relationship stating that a galaxy's recession velocity is directly proportional to its distance from us, written v = H x d.
Dark matter
Matter that neither absorbs, reflects, nor emits light and is inferred only from its gravitational effects; about 27% of the universe's contents.
Dark energy
The unknown driver of the accelerating expansion of the universe, making up roughly 68% of the universe's contents in the current model.
Void
A vast, nearly empty region of space between the filaments and walls of the cosmic web, typically about 150 million light-years across.

Sources & references

  1. Universe Glossary — NASA Science (science.nasa.gov)
  2. Universe: Overview — NASA Science (science.nasa.gov)
  3. What is Dark Energy? Inside Our Accelerating, Expanding Universe — NASA Science (science.nasa.gov)
  4. What is Dark Matter? — NASA Science (science.nasa.gov)
  5. Galaxies: Large Scale Structures — NASA Science (science.nasa.gov)
  6. 26.5 The Expanding Universe, Astronomy 2e — OpenStax, Rice University
  7. 28.3 The Distribution of Galaxies in Space, Astronomy 2e — OpenStax, Rice University
  8. 28.4 The Challenge of Dark Matter, Astronomy 2e — OpenStax, Rice University
  9. 29.5 What Is the Universe Really Made Of?, Astronomy 2e — OpenStax, Rice University
  10. 6.1 Telescopes — OpenStax, Rice University

EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.

Researched 2026-08-21

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