The universe began without a measurable dimension. What came before remains unknown. It is possible there was nothing, or something unknowable. According to theory and faith, there may have been no ‘before’ at all — time itself may have literally begun with this event. Indeed, time and space were generated during the event itself — not as objects, but as properties of the universe.
According to modern cosmology, space did not expand into a pre-existing void; rather, the fabric of space itself stretched, increasing the distance between all points. The space between galaxies, and between each and every object, is steadily expanding at a measurable rate. This can be observed today through the redshift of distant light. One model likens space to a grid or pixel field — a vast chessboard whose squares can be multiplied. As new intervals of space emerge between existing regions, the distance between objects will naturally increase.
Roughly 13.8 billion years ago, the event known as the Big Bang occurred. The Big Bang was not an explosion within space, but an expansion of space itself — a colossal fireball in which all energy and matter were condensed. In one sense, we are still inside the ongoing expansion of that primordial fireball. Some models compare it to a black hole in reverse — a theoretical white hole, from which matter and energy flow outward rather than inward. Indeed, all known energy, matter, and spacetime emerged from such an extremely hot and dense source.
Within the first 10⁻⁴³ seconds, the four fundamental forces of nature — gravity, electromagnetism, the strong nuclear force, and the weak nuclear force — were unified. They behaved as one indistinguishable force under extreme temperature and density. As the universe cooled and energy dropped, the symmetry between them broke — meaning the forces no longer behaved identically. This collapse of symmetry revealed distinct interactions governing different aspects of the physical world. As temperature dropped below 10³² Kelvin, gravity separated, followed by the others.
Space expanded continuously from the moment of the Big Bang onward. This expansion can be readily observed across the cosmos, as the patterns and filaments of galaxies correspond to regions shaped by quantum fluctuations — microscopic density variations that expanded into vast filamentary structures. Indeed, quantum irregularities became the scaffolding for cosmic structure.
Within one second, the universe was filled with quarks, gluons, electrons, positrons, neutrinos, and photons. Matter and antimatter briefly coexisted. Antimatter is composed of particles that mirror ordinary matter in mass but have opposite charge — such as the positron, which is the antiparticle of the electron. When a particle meets its antiparticle, they annihilate in a burst of energy. In the early universe, matter and antimatter were produced in nearly equal amounts, but a slight excess of matter — about one part per billion — survived the mutual annihilation and went on to form all known matter.
Within three minutes, conditions allowed nuclear fusion. Free protons and neutrons collided to form helium-4, along with traces of deuterium, helium-3, and lithium-7. This period of nucleosynthesis ended before heavier elements could form, as temperatures fell rapidly. At this stage, the universe was still opaque — a seething plasma where photons scattered continuously off charged particles, unable to travel far without interaction. Energy densities were dominated by radiation, not matter. In this early era, photons carried the majority of the universe’s energy. These photons were constantly scattered by charged particles in the plasma — primarily free electrons — preventing them from traveling long distances. This made the universe opaque, like a dense fog lit from within. Light could not travel freely. The universe glowed, but nothing was visible. Only when neutral atoms later formed could photons stream freely, giving rise to the cosmic microwave background we observe today.
Gravity had already begun to pull denser regions inward, but it would be hundreds of millions of years before the first stars ignited. The elemental composition of this early cosmos was simple: 75% hydrogen, 25% helium by mass. However, this hydrogen and helium existed in ionised form — as free protons and electrons suspended in a hot, thick plasma. They were not yet whole atoms. Only later, after cooling, would these particles combine into stable, electrically neutral atoms. More complex elements — such as iron, carbon, and uranium — had not yet even begun to form.
At its peak, the early universe reached temperatures around 10³² Kelvin — a trillion trillion degrees. 380,000 years later, the universe cooled below 3000 Kelvin. Electrons slowed enough to bond with atomic nuclei. Neutral hydrogen and helium atoms formed — meaning that electrons bonded to nuclei, resulting in electrically neutral atoms. Once neutral atoms formed, photons could travel unimpeded. This phase is called recombination, marking the universe’s transition from opaque to transparent. It left behind relic radiation — the faint afterglow of that moment — now stretched into the microwave band. This cosmic background radiation, discovered in 1965, can still be observed today.
These variations correspond to differences in matter density and quantum fluctuations within the first moments of expansion. Gravity worked slowly but persistently over eons. Denser regions became gravitational wells, pulling gas inward. Over hundreds of millions of years, gravitational pull intensified the denser regions, while vast voids expanded between them — immense expanses of nearly empty space. One such example is the Boötes Void — a vast region over 300 million light-years in diameter and one of the largest known cosmic voids. Some larger voids, such as the Eridanus Supervoid, may span over one billion light-years. The first cosmic structures — dark matter filaments and baryonic clouds (ordinary matter composed of protons and neutrons) — also took form in this manner. Dark matter filaments formed first, providing the gravitational framework into which baryonic matter later collapsed. Though stars had not yet ignited, the universe’s large-scale structure was thus determined. The modern web of galaxies reflects these primordial variations, which we can trace across billions of light-years.
For millions of years, no new light was produced. This period — known as the cosmic dark age — was marked by slow contraction of hydrogen into potential wells shaped by dark matter halos. In simple terms, gravity was pulling clouds of gas together, slowly setting the stage for the first stars. Photons no longer scattered, yet no stars had begun to shine. Eventually, in regions of sufficient mass, gravitational collapse ignited hydrogen fusion in stellar cores. Around 100–200 million years after the Big Bang, the first stars appeared.
The initial stars, known as Population III — the first in a classification system based on chemical composition — formed from pure hydrogen and helium, with no heavier metals. Models suggest they were massive — between 30 and 300 times the Sun’s mass — and shone intensely for only a few million years. Their ultraviolet radiation began the process of reionisation — the stripping of electrons from neutral hydrogen atoms — which began around 200 million years after the Big Bang and continued for several hundred million years. Though none remain today, the existence of Population III stars is inferred from indirect evidence: the chemical abundance of later stars, the increased transparency of intergalactic space, and high-redshift galaxies observed by the James Webb Space Telescope.
The death of these first stars — in supernovae and hypernovae — scattered carbon, nitrogen, oxygen, and iron into space. From this debris, the second generation of stars began to form. Around 300 million years after the Big Bang, Population II stars emerged — metal-poor, but no longer pristine. Found in galactic halos and globular clusters, these inherited trace amounts of heavy elements from Population III explosions. Later still came the Population I stars, richer in metals, like our Sun. They formed from multiple cycles of stellar birth and death, incorporating heavier elements produced by preceding generations. Population I stars, including the Sun, began forming around 1 billion years later. This progression from pure to enriched stars reflects the chemical evolution of our universe.
By 12 billion years ago, galaxies began to take shape. A galaxy is a gravitationally bound system of stars, gas, dust, and dark matter. They range in size from dwarf galaxies with a few billion stars to giants containing more than a hundred trillion. Gravity guided gas along dark matter filaments into dense regions. As protogalaxies merged and clustered under gravity, angular momentum caused many to flatten into disc-shaped galaxies, much like a spinning cloud flattens under centrifugal forces. In their centres, supermassive black holes formed through gravitational collapse. The Milky Way was formed during this epoch. Its oldest stars, in the galactic halo and bulge, are twelve billion years old. Smaller satellite galaxies were gradually drawn in, and our galactic disc took several billion years to stabilise. The thick disc formed first, while the thin disc — where the Sun resides — formed later, around 8–10 billion years ago.