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 Matter?
Updated by Spaceweathertracker staff ·
An invisible substance that outweighs everything you can actually see — and that no one has ever directly detected.
Dark matter is a form of matter that doesn't emit, absorb, or reflect light — making it completely invisible to telescopes — but whose gravitational pull is essential to how galaxies form, rotate, and hold together. It's thought to make up about 27% of the universe's total mass and energy, roughly five times more than all the ordinary matter (stars, planets, gas, and everything you can see) combined. Despite decades of searching, no dark matter particle has ever been directly detected — its existence is inferred purely from its gravitational effects.
The problem dark matter was invented to solve
Stars orbiting near the edge of a spiral galaxy should, by ordinary physics, move more slowly than stars closer to the crowded, massive center — the same way outer planets in our solar system orbit the Sun more slowly than Mercury does. But when astronomers actually measured galaxy rotation speeds starting in the 1970s (most famously by Vera Rubin), they found something startling: stars at a galaxy's edge orbit almost as fast as stars near its center. The only way to explain that with known physics is if galaxies contain far more mass than what's visible — spread out in a much larger halo than the visible disk of stars.
It's not just galaxy rotation
Rotation curves were the first major clue, but they're far from the only evidence. Gravitational lensing — the bending of light from distant objects by the gravity of matter in front of them — reveals far more mass in galaxy clusters than their visible stars and gas can account for, and the pattern of that bending matches predictions for a large, invisible mass component.
The cosmic microwave background, the afterglow of the Big Bang, contains a precise pattern of tiny temperature variations that depends sensitively on how much total matter existed in the early universe — and the amount of ordinary matter alone doesn't fit the pattern; adding dark matter does, with high precision.
These are independent lines of evidence, discovered by different methods decades apart, and they all point toward the same conclusion — which is a large part of why dark matter is taken so seriously despite never being directly observed.
So what actually is it?
This is the genuinely unresolved part. The leading hypothesis is that dark matter consists of a not-yet-discovered type of particle that interacts with ordinary matter only (or almost only) through gravity — candidates researchers have proposed include WIMPs (weakly interacting massive particles) and axions, among others. Multiple experiments around the world are actively searching for direct evidence of these particles, so far without a confirmed detection. A small minority of physicists instead propose that our understanding of gravity itself needs modification at galactic scales (an idea called MOND), rather than invoking unseen matter — but this approach struggles to explain the full range of evidence as well as dark matter does, particularly at the scale of galaxy clusters and the cosmic microwave background.
How scientists are searching for it
Dark matter searches generally fall into three approaches. Direct-detection experiments, often built deep underground to shield them from cosmic-ray interference, wait for a hypothesized dark matter particle to occasionally collide with an ordinary atomic nucleus and produce a tiny, measurable signal. Indirect-detection experiments instead look for the byproducts that would result if dark matter particles occasionally collide with and destroy each other — unusual gamma rays or cosmic-ray particles arriving from regions expected to be dense with dark matter, like the center of our galaxy. And collider experiments, like those at the Large Hadron Collider, attempt to actually produce dark matter particles by smashing ordinary particles together at extremely high energies. None of the three approaches has yet produced a confirmed detection, but each rules out large swaths of possible dark matter candidates with every new result, steadily narrowing where the answer could be hiding.
What scientists know
Multiple independent lines of evidence — galaxy rotation curves, gravitational lensing, and the cosmic microwave background — consistently point to roughly five times more gravitating mass in the universe than visible matter can account for.
What remains uncertain
What dark matter actually is remains unknown. No dark matter particle has been directly detected despite decades of dedicated search experiments.
Common misconceptions
✗ Dark matter and dark energy are the same thing.
They're entirely different. Dark matter pulls things together via gravity, like ordinary matter. Dark energy pushes space apart, driving the universe's accelerating expansion.
✗ Dark matter is just matter we haven't found yet, like a rogue planet.
The evidence points to dark matter being a fundamentally different kind of particle, not ordinary hidden matter (like dim stars or planets) — the cosmic microwave background and other evidence rule out ordinary matter being the culprit.
Related questions
Is dark matter dangerous?
No. Dark matter interacts so weakly with ordinary matter that it passes through planets and people constantly without any detectable effect.
How much dark matter is in the Milky Way?
Estimates suggest our galaxy's dark matter halo contains several times more mass than all its visible stars combined, extending well beyond the visible disk.