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
Are There Parallel Universes?
Updated by Spaceweathertracker staff ·
Several distinct, serious physics ideas point toward something like a multiverse — but none of them currently has direct evidence, and they don't all mean the same thing.
Nobody knows. "Parallel universes" or "the multiverse" isn't one single theory — it's an umbrella term covering several genuinely different hypotheses that arise from different areas of physics. Some are considered by physicists to be natural, if unconfirmed, consequences of well-tested theories. None currently has any direct observational evidence, and some may be untestable in principle. This puts the multiverse in a different category from more directly evidenced ideas like dark matter — it's real, serious physics, but genuinely speculative.
Not one idea — several different ones
A common source of confusion is that "multiverse" gets used for at least three distinct concepts, each with a different physical basis:
1. The infinite universe
If space is truly infinite (a real, live possibility discussed on our Is the Universe Infinite? page) and matter is arranged similarly on average everywhere, then simple statistics suggest that far enough away, particle arrangements should eventually repeat — including, in principle, arrangements resembling our own observable universe. This follows logically from a genuinely infinite, statistically uniform universe, but it isn't something that can ever be directly observed, since those regions lie far beyond our cosmic horizon.
2. Eternal inflation and bubble universes
Cosmic inflation, the well-supported theory of the universe's rapid early expansion, has a variant called eternal inflation, in which inflation never fully stops everywhere — it keeps happening in some regions while ending in others, constantly spawning new, causally disconnected "bubble" universes, each potentially with different physical properties. This isn't a fringe idea; it emerges naturally from some well-regarded inflationary models. But it currently has no direct observational test, and some physicists argue it may not be testable even in principle.
3. The many-worlds interpretation of quantum mechanics
In quantum mechanics, particles can exist in a superposition of multiple possible states until measured. The many-worlds interpretation proposes that every possible outcome of a quantum event actually happens, each in its own separate, branching universe — rather than a single outcome being randomly "selected." This is a serious, actively debated interpretation among physicists, not a fringe idea, but it remains one of several competing interpretations of quantum mechanics, and there's no experiment that distinguishes it from the alternatives — interpretations, by design, all predict the same observable results.
What scientists know
Cosmic inflation and quantum superposition are both extremely well-supported by observation and experiment. The mathematical extensions of these theories that lead to multiverse concepts are legitimate, studied consequences of that underlying physics.
What remains uncertain
Whether any version of the multiverse is physically real is unknown, and may be unknowable — several versions make no testable predictions that differ from a single-universe model, which is a genuine limitation, not just a temporary gap in technology.
Common misconceptions
✗ Scientists believe in parallel universes the same way they believe in dark matter.
Dark matter has multiple independent lines of observational evidence. Multiverse hypotheses currently have none — they're taken seriously because they follow from well-tested theories, which is a different, weaker kind of support.
✗ Parallel universes mean there's another you living a different life somewhere.
This specific, popular framing mixes together the infinite-universe idea and many-worlds quantum mechanics, which are actually different hypotheses with different physical bases — neither is confirmed, and they don't necessarily imply what science fiction usually depicts.
Related questions
Could we ever visit a parallel universe?
Under every current multiverse hypothesis, other universes (if they exist) would be causally disconnected from ours — meaning no signal, matter, or travel of any kind could pass between them. This isn't a matter of insufficient technology; it follows from the physics of the proposals themselves.
Do physicists take the multiverse seriously?
Many do take it seriously as a mathematical consequence of well-supported theories, while remaining clear that it's unconfirmed. It's a genuine, active area of theoretical physics — not a fringe topic — but distinct from settled science.