Look up at night! ⭐
The sky is big, full of stars, the Moon, and distant suns.
Long, long ago, it all began as one tiny hot dot, and then it grew and grew.
No one knows what was before that dot, and the sky keeps its secret for now.
How Big Is the Sky?
The universe is everything there is: stars, planets, and space between them. The part we can see is about 93 billion light-years across. A light-year is how far light travels in one year.
What Is Outside It?
No one can see outside it, and there may not be an outside at all. Think of a balloon's surface: you can walk around it and never find an edge. The universe may be like that.
How Did It Start?
About 13.8 billion years ago, everything was squeezed into one tiny, hot dot. Then the dot began to grow fast, and it is growing. Scientists call that growth the Big Bang.
What Came Before the Start?
Nobody knows. Maybe time began with the Big Bang, so there was no before. Scientists have ideas, but none are proven. "We do not know" is an honest answer.
A Sky Too Big to Picture
The universe is everything that exists. Every star you see, every planet, every speck of dust, and all the empty space in between is part of it. It is bigger than your brain can easily picture, which is part of the fun.
How Big Is the Part We Can See?
Scientists can only see a part of the universe, the part whose light has had time to reach us. That part is called the observable universe, and it is about 93 billion light-years across. A light-year is the distance light travels in one year, about 9.5 trillion kilometers, so 93 billion of them is a number so large it stops feeling real.
What Is Outside It?
Here is the strange part: there may be no outside at all. The universe might go on forever, or it might curve back on itself the way the surface of Earth does, so you could travel forever and never reach an edge. Either way, the part beyond what we can see is hidden from us, not because it is far in an ordinary sense, but because its light has not had time to reach us, and the expansion of space may keep it that way forever.
How Did It All Start?
About 13.8 billion years ago, everything in the universe was squeezed into a tiny, unimaginably hot and dense state. Then it began to expand and cool, and it has been expanding ever since. Scientists call this beginning the Big Bang, but it was not an explosion in space. It was the expansion of space itself, like the surface of a balloon stretching as you blow it up.
What Came Before the Beginning?
This is the question that stumps even the experts, and that is okay. Maybe time itself started with the Big Bang, which would mean there was no "before" at all. Or maybe our universe came from something else, like a previous universe that collapsed, or a tiny quantum flutter. Scientists have ideas, but no one has proof yet. "We do not know" is a real answer, and it is the reason scientists keep looking.
The Part We Can See
When astronomers say the universe is 93 billion light-years across, they do not mean the whole universe. They mean the observable universe, the spherical region centered on Earth from which light has had time to reach us since the Big Bang. Its radius, about 46.5 billion light-years, is called the particle horizon, and it marks the edge of what we can ever observe, not the edge of what exists.
Why 93 Billion When the Universe Is Only 13.8 Billion Years Old?
This looks like a contradiction, and resolving it is one of the most satisfying moments in cosmology. The light from the most distant galaxies we see left them 13.8 billion years ago, but space itself has been expanding the whole time that light was traveling. Those galaxies are now about 46.5 billion light-years away, even though their light traveled for only 13.8 billion years. Expansion stretches the distance behind the light's back.
Is There an Edge?
Probably not, at least not one anyone could reach. If the universe is infinite, it simply has no edge, and "outside" is a word without a referent. If it is finite but curved, like the three-dimensional version of the surface of a sphere, then traveling in a straight line would eventually bring you back to where you started, the way a ship sailing around Earth returns home without ever meeting an edge. Either way, there is no wall, no fence, and no place where space stops.
The Hot Beginning
The Big Bang is not a theory about an explosion. It is the observed fact that the universe was once hotter and denser, and has been expanding and cooling for about 13.8 billion years. Three lines of evidence support it: galaxies are moving away from each other, the leftover glow of the hot early universe (the cosmic microwave background) still fills the sky at a chilly 2.7 degrees above absolute zero, and the proportions of light elements like hydrogen and helium match what a hot early universe would have cooked up in its first few minutes, a prediction made before the measurements existed and confirmed by observations of ancient gas clouds that have barely changed since then.
What Caused the Big Bang?
Here honesty matters more than a tidy story. Nobody knows. One possibility is that time itself began at the Big Bang, so asking what came before is like asking what is north of the North Pole: the question sounds sensible but has no answer. Another possibility is that our universe budded off from something else, perhaps through cosmic inflation, a runaway expansion that may still be spawning universes elsewhere, or through the collapse and rebound of a previous universe. These ideas are mathematically serious and experimentally unproven. That gap is not a failure, it is the frontier.
The Particle Horizon
The figure of 93 billion light-years is the diameter of the observable universe, twice the current proper distance to the particle horizon, about 46.5 billion light-years. It is a statement about our observational limits, not a measurement of the whole. The whole universe may be infinite, in which case its size is not a number at all, or it may be finite but vastly larger than the observable patch, with the excess hidden behind the horizon forever. Distinguishing those cases from inside is impossible in principle, because no signal from beyond the horizon can ever reach us. And because the horizon is a limit on observation rather than a physical barrier, the galaxies beyond it are not gone or special in any way, they are simply too far for their light to have reached us yet, and as the universe ages the horizon will keep receding, bringing new galaxies into view even as the accelerating expansion carries others permanently out of reach.
Expansion, Not Explosion
The Big Bang was not matter exploding into preexisting space. It was space itself expanding, carrying galaxies apart the way raisins in rising dough move apart without moving through the dough. That is why distant galaxies can recede from us faster than light without violating relativity: nothing is moving through space faster than light, space itself is stretching, and relativity sets no speed limit on the stretching of space. The recession velocity follows Hubble's law, at least locally.
The Hubble constant, currently measured at roughly 67 to 73 kilometers per second per megaparsec depending on the method, is itself a live controversy: early-universe measurements and late-universe measurements disagree, a mismatch called the Hubble tension that may point to new physics or to subtle measurement errors nobody has found yet.
Three Pillars of the Hot Big Bang
The hot Big Bang model rests on three independent pillars, which is why it survives despite its unanswered questions. First, the expansion itself, measured by Hubble and refined by a century of galaxy surveys. Second, the cosmic microwave background, a nearly uniform glow at 2.725 Kelvin across the whole sky, the cooled remnant of the universe's hot youth, mapped in exquisite detail by the COBE, WMAP, and Planck satellites. Third, Big Bang nucleosynthesis: the observed abundances of hydrogen, helium, and lithium match the predictions for nuclear reactions in the first few minutes, to a precision no competing model has matched.
What About Before?
General relativity breaks down at the singularity, the point where its equations return infinities, so the theory that describes the expansion cannot describe the beginning itself. Candidate quantum-gravity ideas fill the gap with very different pictures. The Hartle-Hawking no-boundary proposal treats time as behaving like a spatial dimension near the beginning, so there is no sharp starting edge, only a smooth rounding off, like asking where the South Pole's south is. Cyclic models, including the ekpyrotic scenario, have universes colliding and rebounding in an endless series. The honest summary is that physics currently has several candidate answers, zero confirmed ones, and no experiment yet designed that could settle the question. A student who says "we do not know" about the cause of the Big Bang is not behind. They are exactly where the science is.
A Child's Four Questions, Answered Honestly
How big is the universe, what is outside it, how did it start, and where did the Big Bang come from? These are four different questions, and they have four different kinds of answers: one measured, one possibly meaningless, one well established, and one frankly unknown. Conflating them is how confident nonsense gets written, so it is worth keeping them apart.
The Measured Answer: 93 Billion Light-Years
The observable universe, the region from which light has had time to reach us, is about 93 billion light-years in diameter. That number sounds impossible beside the universe's age of 13.8 billion years, and the resolution is expansion: the space between us and the most distant visible galaxies has stretched while their light was in transit, so those galaxies now sit roughly 46.5 billion light-years away even though we see them as they were 13.8 billion years ago. The observable universe contains on the order of two trillion galaxies, and its volume grows every second as the particle horizon recedes, though the galaxies crossing it are lost to us, their light redshifted beyond detection. Pause on that. To put the scale in human terms, if the Sun were shrunk to the size of a grain of sand, the nearest star would still be about seven kilometers away, the Milky Way would stretch across a continent, and the observable universe would be a sphere so vast that walking its diameter at a steady pace would take longer than the age of the universe itself by a factor of millions.
The Possibly Meaningless Question: What Is Outside?
If the universe is infinite, and current measurements are consistent with a flat, possibly infinite cosmos, then there is no outside, and the question dissolves the way "what is north of the North Pole" dissolves. If it is finite but unbounded, curved back on itself like a higher-dimensional sphere, then traveling far enough would return you home, and again there is no outside to point at. There is a third sense of "outside" worth naming: the multiverse proposals, eternal inflation's bubble universes or the many-worlds of quantum mechanics, which posit realities beyond our own. These are not observations, they are extrapolations of theories that work inside our universe, and they should be labeled as such whenever they appear in a popular article, including this one.
The Established Answer: A Hot, Dense Beginning
The Big Bang is among the best-supported claims in science, and it is worth saying precisely what it claims. It claims that the universe was once hotter and denser and has expanded and cooled for 13.8 billion years. It does not claim that everything exploded out of a point into empty space, and it does not claim to know what, if anything, preceded that expansion. The evidence, expansion, the 2.725 Kelvin microwave background, and the light-element abundances, is independent, quantitative, and has survived decades of attempts to break it. The acceleration of the expansion, discovered in 1998 and attributed to dark energy, adds a strange coda: distant galaxies are not merely receding, they are receding ever faster, which means the observable universe's contents are thinning out over cosmic time, and far-future observers may see a much emptier sky than we do.
The Unknown: What Caused It?
Here the honest answer is a short one: nobody knows, and it is not clear what kind of answer would even count. If time emerged with the Big Bang, then "before" is undefined, and the search for a cause may be a category error, a question grammar smuggles past physics. If time extends further back, then candidates compete without a referee: eternal inflation, cyclic cosmologies, quantum creation from a vacuum state, the no-boundary proposal. Each has serious advocates and none has a confirmed prediction. The Hubble tension, the disagreement between early and late measurements of the expansion rate, is a reminder that even the established parts of the story still contain cracks, and those cracks are where the next breakthrough will come from. The universe is big enough to humble every answer, and the most reliable thing a parent can tell a child is that the biggest questions are still open, and that "we do not know" is not a shrug, it is an invitation.
Sources
- Planck Collaboration. "Planck 2018 results. VI. Cosmological parameters." Astronomy and Astrophysics, 2020.
- NASA / WMAP Science Team. "Cosmic Microwave Background." NASA Wilkinson Microwave Anisotropy Probe.
- Guth, Alan H. "Inflationary universe: A possible solution to the horizon and flatness problems." Physical Review D, 1981.
- Hartle, J. B. and Hawking, S. W. "Wave function of the Universe." Physical Review D, 1983.
- Riess, A. G. et al. "A Comprehensive Measurement of the Local Value of the Hubble Constant." The Astrophysical Journal, 2022.
- Steinhardt, P. J. and Turok, N. "A Cyclic Model of the Universe." Science, 2002.
- Weinberg, Steven. The First Three Minutes. Basic Books, 1977.