Space Education
The Universe
Updated by Spaceweathertracker staff ·
Your gateway to understanding everything from the edge of the observable universe down to Earth's own atmosphere — from distant galaxies to the solar storms that shape our skies.
What is the universe?
The universe is everything that exists — every galaxy, star, planet, and particle, along with the space between them and the physical laws that govern how they behave. It began roughly 13.8 billion years ago in an extremely hot, dense state and has been expanding and cooling ever since, an event generally called the Big Bang. That expansion is still happening today, and — as covered on What Is Dark Energy? — it's actually speeding up rather than slowing down.
Most of what the universe is made of isn't visible at all. Ordinary matter — everything in stars, planets, gas, and every person who has ever lived — makes up only about 5% of its total mass and energy. Roughly 27% is dark matter, an invisible substance detected only through its gravity, and the remaining 68% is dark energy, whatever is driving that accelerating expansion. Both remain among the biggest open questions in physics.
On the largest scales, matter isn't spread evenly through space. Galaxies cluster into groups, clusters, and vast filament-like superclusters, separated by enormous, nearly empty voids — a pattern astronomers sometimes call the cosmic web. Our own galaxy, the Milky Way, is one of an estimated hundreds of billions within just the part of the universe we're able to see, which is itself only a fraction of the whole — see The Observable Universe for why we can only ever see part of it, and How Big Is the Universe? for what that actually means in numbers.
Scientists study the universe mainly by observing light — and other forms of electromagnetic radiation — arriving from distant objects, since almost everything we know about space beyond our own solar system comes from analyzing that light rather than visiting it directly. One especially important signal is the cosmic microwave background, the faint afterglow of the Big Bang itself, still detectable in every direction and carrying precise information about the universe's age, composition, and large-scale shape. Where the evidence runs out — what came before the Big Bang, what dark matter and dark energy actually are, whether space is truly infinite, how it will all end — is exactly the territory the nine questions below dig into, each one clearly separating what's actually confirmed from what's still genuinely unknown.
The Universe: Big Questions
The biggest questions in cosmology — answered clearly.
How Big Is the Universe?
The observable universe — the part we can actually see and measure — is roughly 93 billion light-years across. That's the region from which light has had time to reach us since the universe became transparent.
That number isn't the size of the entire universe, though. Space itself has expanded while light from distant galaxies traveled toward us, so extremely distant objects are now much farther away than a simple age-times-speed-of-light calculation would suggest.
Scientists don't currently know the total size of the universe. It could be vastly larger than the observable universe — and it may even be infinite.
What we know
- The observable universe is well-measured at roughly 93 billion light-years across.
- The universe is approximately 13.8 billion years old.
What we don't yet know
- The total size of the universe beyond what we can observe.
Is the Universe Infinite?
We don't know. Observations show the observable universe is extremely large and that space is very close to geometrically flat on the scales we can measure — but a flat universe doesn't automatically mean an infinite one.
The universe could be infinite, or it could be finite but so enormous that we simply can't detect its overall shape from our observable region. Scientists therefore distinguish carefully between what we can observe and what the entire universe actually contains.
What we don't yet know
- The total size and global topology of the universe.
What Was Before the Big Bang?
We don't currently know whether the question "what happened before the Big Bang?" even has a meaningful answer in the conventional sense.
The Big Bang model describes the early hot, dense, expanding universe — it doesn't necessarily describe an ordinary explosion happening at one location in pre-existing space. Our current physical theories become incomplete when we try to describe the earliest moments, particularly where quantum mechanics and gravity must be treated together.
There are hypotheses involving inflation, quantum cosmology, and bouncing universes, among others — but none has been established as the definitive answer.
What we don't yet know
- What, if anything, preceded the earliest physical state described by current cosmology.
What Is Dark Matter?
Dark matter is the name scientists give to an unseen form of matter whose presence is inferred from its gravitational effects. It doesn't appear to emit, absorb, or reflect light the way normal matter does, so we can't see it directly with ordinary telescopes.
Its gravitational effects help explain the motions of stars and galaxies, the behavior of galaxy clusters, and gravitational lensing. Current estimates suggest ordinary matter makes up about 5% of the universe, dark matter about 27%, and dark energy about 68%.
Scientists still don't know exactly what dark matter is made of.
What we know
- Dark matter's gravitational effects are strongly supported by multiple independent lines of observation.
What we don't yet know
- The fundamental nature of dark matter — what it's actually made of.
What Is Dark Energy?
Dark energy is the name scientists give to whatever is responsible for the universe's observed accelerated expansion. Astronomers discovered in the late 1990s that cosmic expansion is speeding up rather than simply slowing under gravity.
The leading explanation is a cosmological constant associated with the energy of empty space, but scientists don't yet know what dark energy fundamentally is. Other possibilities include evolving fields or modifications to our understanding of gravity itself. It's estimated to account for roughly 68% of the universe's mass-energy budget.
What we don't yet know
- The fundamental physical nature of dark energy.
What Is the Observable Universe?
The observable universe is the portion of the universe from which light or other information has had time to reach Earth since the early universe. It's currently estimated to be roughly 93 billion light-years across.
The observable universe is centered on the observer — a civilization in a distant galaxy would have its own observable universe, centered on its own location. It's therefore not necessarily the entire universe.
Explore the full explanation →Are Parallel Universes Real?
We don't know. The idea of parallel or multiple universes appears in several theoretical frameworks in physics and cosmology, including some interpretations of quantum mechanics and certain models of cosmic inflation.
However, there is currently no direct observational evidence confirming that other universes exist. Parallel universes should be understood as a theoretical possibility being actively studied, not as an established discovery.
What we don't yet know
- Whether other universes exist in any form.
How Will the Universe End?
The ultimate fate of the universe depends on how cosmic expansion, dark energy, gravity, and the universe's contents behave over extremely long timescales.
The leading scenario under the simplest interpretation of today's observations is continued expansion, eventually leading to a very cold, dilute universe sometimes called the "Big Freeze" or heat death. Other possibilities — including a Big Rip or Big Crunch — have been studied under different assumptions about dark energy and cosmology. The exact fate isn't known with certainty.
Explore the full explanation →Are We Alone?
We don't know. Earth is currently the only world known to host life. However, astronomers have discovered thousands of planets orbiting other stars, including worlds that may have environments capable of supporting some of the conditions associated with life.
Scientists are searching for signs of life through observations of planets, moons, atmospheres, radio signals, and other potential biosignatures. So far, there is no confirmed evidence of extraterrestrial life. This remains one of the biggest unanswered questions in science.
What we don't yet know
- Whether life exists anywhere beyond Earth.
What do you want to explore?
Looking for what's happening right now rather than the big evergreen questions? Visit Space News for the latest discoveries, or Galaxies & Nebulas to go deeper on galactic structure. If you'd rather look up than read, Sky Tonight shows what's actually visible from your location right now.
The Universe
What Is Dark Energy?
Updated by Spaceweathertracker staff ·
It makes up more of the universe than everything else combined, and we still don't know what it actually is.
Dark energy is the name given to whatever is causing the expansion of the universe to accelerate rather than slow down over time. It's estimated to make up about 68% of the total mass-energy of the universe — more than dark matter (27%) and ordinary matter (about 5%) combined. Unlike dark matter, which pulls things together through gravity, dark energy appears to push space apart. Its exact physical nature remains one of the biggest open questions in physics.
How we found out expansion was speeding up
For most of the 20th century, cosmologists expected the universe's expansion to be gradually slowing down, since gravity should be pulling everything back together. That assumption was overturned in 1998, when two independent research teams — the Supernova Cosmology Project and the High-Z Supernova Search Team — measured the distances to dozens of Type Ia supernovae: a type of exploding star with a highly consistent, predictable peak brightness, making them useful as "standard candles" for measuring cosmic distances with real precision. Both teams found the same surprising result independently: distant supernovae were dimmer, and therefore farther away, than a slowing expansion would predict. The simplest explanation was that expansion itself is accelerating, not decelerating. This discovery earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics, and it has since been independently confirmed by completely different methods — most notably by detailed measurements of the cosmic microwave background and by large-scale surveys mapping how galaxies cluster across the universe.
The leading explanation: the cosmological constant
The simplest model that fits the data treats dark energy as a cosmological constant — a fixed amount of energy built into empty space itself, sometimes described loosely as "vacuum energy." As space expands, more space means more of this constant energy, which pushes expansion to accelerate further, creating a runaway effect that becomes more dominant as the universe grows and matter thins out. This model (often called Lambda-CDM, combining a cosmological constant, denoted by the Greek letter lambda, with cold dark matter) fits an enormous range of cosmological observations remarkably well — everything from the fine structure of the cosmic microwave background to the large-scale distribution of galaxies. But a cosmological constant is a mathematical description of the effect, not an explanation of its underlying physical cause; it tells us how dark energy behaves, not what it actually is.
Why physicists find this deeply puzzling
Quantum field theory offers a natural candidate for vacuum energy — the idea that even seemingly empty space is filled with fleeting quantum fluctuations that carry real energy. But when physicists calculate the expected value of that energy from quantum theory and compare it to the actual measured value of dark energy, the two numbers disagree by an almost incomprehensible margin — by some estimates, on the order of 10^120 times. This mismatch, sometimes called the "cosmological constant problem" or even "the worst theoretical prediction in the history of physics," is considered one of the largest unresolved discrepancies between theory and observation in all of physics. Some researchers propose dark energy isn't constant at all, but a dynamic field that changes over time — often called "quintessence," borrowing a term from ancient and medieval philosophy for a fifth fundamental element. Current data doesn't yet distinguish clearly between a true constant and a slowly evolving field.
Could it be gravity that's wrong instead?
A smaller but genuinely active line of research asks whether dark energy is real at all, or whether cosmic acceleration is actually a sign that our understanding of gravity itself needs revision on the very largest scales. General Relativity has passed every precision test performed so far — including tests within our own solar system and observations of merging black holes — but some physicists have proposed modified gravity theories that could reproduce the observed acceleration without invoking a separate dark energy component. So far, none of these alternative theories fits the full range of cosmological data as well as standard dark energy does, but they remain part of the genuine scientific conversation rather than a settled, closed question.
What scientists know
The universe's expansion is accelerating — this is well established from multiple independent methods (supernovae, the cosmic microwave background, and large-scale galaxy surveys). Dark energy's approximate share of the universe's total energy content (about 68%) is measured with good precision.
What remains uncertain
What dark energy physically is remains unknown. Whether it's truly constant over cosmic time or slowly changing is an active area of research, and the huge mismatch between theoretical predictions and the observed value is unresolved.
Common misconceptions
✗ Dark energy and dark matter are the same mysterious substance.
They behave in opposite ways — dark matter's gravity pulls matter together, while dark energy pushes space apart — and they're generally treated as distinct phenomena in current models.
Related questions
Will dark energy ever run out?
If it's truly a constant, it won't — it would keep driving accelerating expansion indefinitely. If it's a dynamic field, its future behavior is genuinely uncertain and is an active research question.
Does dark energy affect anything on Earth or in the solar system?
No — its effect is only significant at the vast scales between galaxies and galaxy clusters; it has no measurable effect on gravitationally bound systems like the solar system.
When was dark energy discovered?
The accelerating expansion it's inferred from was discovered in 1998, though the term "dark energy" and serious theoretical study of what it might be only developed in the years that followed.
How do scientists measure dark energy if they can't detect it directly?
Indirectly, through its effects: how the brightness of distant standard-candle supernovae compares to their redshift, subtle patterns in the cosmic microwave background, and how galaxy clustering has evolved over cosmic time. All three independent methods converge on the same roughly 68% figure.