The Universe
13.8 billion years in the making — from the Big Bang to the galaxies of today, the story of the cosmos, every milestone sourced.
A timeline of the universe itself, from the Big Bang some 13.8 billion years ago to the vast web of galaxies we see today. It traces the first light of the cosmic microwave background, the birth of the first stars and galaxies, the reionization of the cosmos, the assembly of our own Milky Way, the acceleration driven by dark energy, and the formation of the Sun, the Earth, and the Moon. Every event is backed by content-verified sources from NASA and the European Space Agency.
Events
- 13.8 billion years agoReputable sourceWell documented
The Big Bang
About 13.8 billion years ago the universe began as an unimaginably hot, dense state and started to expand. In the tiniest fraction of a second it ballooned faster than the speed of light in an episode called cosmic inflation. A second later it was a searing 'soup' of light and particles at some 10 billion degrees Celsius; over the next few minutes the first atomic nuclei formed.
Why it matters: The Big Bang is the beginning of space, time, matter, and energy — the origin of everything that would eventually become stars, galaxies, planets, and life.
Sources- NASA Science. The Universe — Overview · reference
- ~380,000 years after the Big BangReputable sourceWell documented
The Cosmic Microwave Background: First Light
For its first few hundred thousand years the universe was an opaque fog of particles. About 380,000 years after the Big Bang it had cooled enough for electrons and nuclei to join into the first atoms — the epoch of recombination — and light was suddenly free to travel. That first light, stretched by the expanding universe into microwaves, still bathes the sky today as the cosmic microwave background.
Why it matters: The cosmic microwave background is the oldest light we can see. Mapped in detail by NASA's WMAP, it pins the age of the universe at 13.8 billion years and reveals that ordinary atoms make up only about 5% of the cosmos, with dark matter and dark energy the rest.
- ~13.5 billion years agoReputable sourceEstimated
The First Stars
After recombination the universe entered the 'cosmic dark ages,' with no shining stars. A few hundred million years after the Big Bang, gravity pulled clouds of hydrogen and helium together until they ignited as the first stars. Made almost entirely of those two lightest elements, these first stars forged the first heavier elements in their cores.
Why it matters: The first stars ended the darkness and began enriching the universe with the elements — carbon, oxygen, iron — that make up planets and living things. Astronomers have not yet directly observed them, so their exact timing is still being pinned down.
Sources- NASA Science. Webb — The Early Universe · reference
- ~13.4 billion years agoReputable sourceWell documented
The First Galaxies
Within the first billion years, stars gathered by gravity into the first galaxies. The James Webb Space Telescope, built to peer into this era, has detected galaxies that existed just a few hundred million years after the Big Bang — one, JADES-GS-z13-1, seen as it was only about 330 million years after the beginning.
Why it matters: Galaxies are the universe's basic building blocks of structure. Webb's discovery that large galaxies formed surprisingly early is reshaping our understanding of how the cosmos assembled.
Sources- NASA Science. Webb — The Early Universe · reference
- ~13 billion years agoReputable sourceWell documented
The Epoch of Reionization
The intense radiation from the first stars and galaxies stripped electrons back off the hydrogen atoms filling space, a process called reionization. Over hundreds of millions of years it burned away the last of the cosmic fog, so that by roughly a billion years after the Big Bang the universe had become transparent to light, as it is today.
Why it matters: Reionization was the last great transformation of the early universe, marking the point when the cosmos lit up and cleared, allowing starlight to travel freely across it.
Sources- NASA Science. Webb — The Early Universe · reference
- ~13 billion years agoReputable sourceWell documented
The Birth of the Milky Way
Our own galaxy began taking shape very early. Measurements from the European Space Agency's Gaia mission show that the Milky Way's thick disc started forming about 13 billion years ago — only around 0.8 billion years after the Big Bang. Over the following billions of years the galaxy grew by pulling in and merging with smaller galaxies, including one called Gaia-Sausage-Enceladus about 10 billion years ago.
Why it matters: The Milky Way is our cosmic home, and mapping how it assembled from ancient stars and galactic collisions tells us where the Sun, the Earth, and we ourselves come from.
- ~5 billion years agoReputable sourceWell documented
Dark Energy and the Accelerating Universe
For most of cosmic history the pull of gravity slowed the universe's expansion. But in the 1990s, studies of distant exploding stars revealed that the expansion is instead speeding up. The cause was named dark energy — a mysterious, repulsive influence that makes up roughly 70% of the universe and, over the last several billion years, has come to drive its accelerating growth.
Why it matters: The discovery of cosmic acceleration, honored with the 2011 Nobel Prize in Physics, revealed that most of the universe is made of something no one yet understands — one of the deepest open questions in science.
- ~4.6 billion years agoReputable sourceWell documented
The Birth of the Solar System
About 4.6 billion years ago, a dense cloud of interstellar gas and dust — part of a larger nebula — collapsed, perhaps triggered by the shockwave of a nearby exploding star. It flattened into a spinning disk; at the center, gravity packed material together until hydrogen began to fuse, and the Sun was born, taking up more than 99% of the matter. The leftover disk clumped into planets, moons, and smaller bodies.
Why it matters: This ordinary event around one young star produced the Sun and its planets — including the Earth, the only place life is known to exist.
Sources- NASA Space Place. How Did the Solar System Form? · reference
Related timelines- Evolution of Life on Earth → — Where the story of life begins
- ~4.5 billion years agoReputable sourceWell documented
Earth and the Moon
The young Earth finished forming about 4.5 billion years ago. Around the same time, according to the leading theory, a Mars-sized body — nicknamed Theia — collided with the Earth, flinging molten and vaporized rock into orbit. That debris quickly gathered together to form the Moon.
Why it matters: The giant impact gave Earth its large Moon, which stabilizes the planet's tilt and drives the tides — conditions that helped make the Earth hospitable to life.
Sources- NASA Science. Moon Formation · reference
- TodayReputable source · 2 sourcesWell documented
The Universe Today
Some 13.8 billion years after it began, the observable universe contains an estimated two trillion galaxies, each with billions of stars, woven into a vast web of filaments and voids. It is still expanding — and, driven by dark energy, expanding ever faster. Everything we can see is made of ordinary matter that accounts for only about 5% of the whole; the rest is invisible dark matter and dark energy.
Why it matters: From a hot, formless beginning the cosmos has built the galaxies, stars, and worlds we see today — and given rise, on at least one small planet, to beings able to look back and reconstruct its history.
SourcesRelated timelines- Space Exploration → — How we set out to explore it