Scientists believe the universe began about 13.8 billion years ago in an event called the Big Bang . This was not an explosion in space — it was an expansion of space itself from an incredibly hot, dense single point . In less than a second , the universe grew from smaller than an atom to a vast , seething fireball of energy .
✨ One of the strongest pieces of evidence for the Big Bang is the Cosmic Microwave Background — a faint glow of heat radiation that fills the entire sky. It is the afterglow of the Big Bang's immense heat , now cooled over billions of years to just a whisker above absolute zero .
📷 ESA and the Planck Collaboration · CC BY 4.0A common mistake is to imagine the Big Bang as a bomb going off somewhere in empty space . But there was no 'outside' for it to explode into — space , time , and everything else all began at that moment . It is better to picture the universe like the surface of a balloon being blown up, with every point moving away from every other point as the whole thing stretches .
How do we know the universe is expanding ? In the 1920s the astronomer Edwin Hubble noticed that distant galaxies are racing away from us, and the further away a galaxy is, the faster it flees . If everything is flying apart today , then long ago it must all have been packed together — which points straight back to the Big Bang .
In the first three minutes after the Big Bang , the universe was hot enough for nuclear fusion to make the simplest elements : hydrogen and helium . These two gases are still the most common elements in the universe today . After about 380,000 years , the universe had cooled enough for atoms to form , and it became transparent — light could travel freely for the first time .
🧩 Put these events in the history of the universe in the right order.
Complex life appears on Earth (about 13.5 billion years later) ? ⤒ ↑ ↓ ⤓
Our Sun and solar system form (about 9 billion years later) ? ⤒ ↑ ↓ ⤓
The first stars ignite (about 100–200 million years later) ? ⤒ ↑ ↓ ⤓
The first atoms form (about 380,000 years later) ? ⤒ ↑ ↓ ⤓
The Big Bang — the universe begins ? ⤒ ↑ ↓ ⤓
✨ Every chemical element heavier than hydrogen and helium — including the carbon , oxygen , and iron in your body — was forged inside the nuclear furnaces of stars and scattered into space when those stars exploded . You are literally made of star dust .
The very first stars were born from clouds of hydrogen and helium pulled together by gravity . Inside them, fusion forged heavier elements like carbon and oxygen . When these giant early stars exploded as supernovae , they scattered those elements into space , where they later helped build new stars , planets — and eventually living things. We truly are made of the ashes of ancient stars .
📷 NASA ESA · Public domain🔗 Match each term to what it means.
The faint afterglow of the Big Bang's heat A mysterious force driving the universe to expand faster Invisible matter that holds galaxies together The moment space and time began, 13.8 billion years ago
The faint afterglow of the Big Bang's heat A mysterious force driving the universe to expand faster Invisible matter that holds galaxies together The moment space and time began, 13.8 billion years ago
The faint afterglow of the Big Bang's heat A mysterious force driving the universe to expand faster Invisible matter that holds galaxies together The moment space and time began, 13.8 billion years ago
Cosmic Microwave Background
? The faint afterglow of the Big Bang's heat A mysterious force driving the universe to expand faster Invisible matter that holds galaxies together The moment space and time began, 13.8 billion years ago
Astronomers have discovered that the universe is not only expanding — the expansion is speeding up. Something called dark energy seems to be pushing everything apart . Nobody yet knows what dark energy is. Together with dark matter (invisible matter that holds galaxies together ), these mysterious things make up about 95% of everything in the universe , leaving only 5% as the ordinary matter we can see and touch .
📷 Photo credit: Jon A. Morse (STScI) and NASA Investigating team: William P. Blair · Public domain💡 The universe is so old that imagining its timeline is very hard . A useful scale : if the entire history of the universe were squeezed into one calendar year , modern humans would appear in the final 14 seconds of 31 December !
🗂️ Sort each part of the universe into ordinary matter or the mysterious 'dark' parts.
Ordinary matter we can see Mysterious 'dark' parts
Ordinary matter we can see Mysterious 'dark' parts
Dark matter holding galaxies together
? Ordinary matter we can see Mysterious 'dark' parts
Dark energy pushing space apart
? Ordinary matter we can see Mysterious 'dark' parts
✍️ Fill in the facts about the universe.
The universe began about 13.8 billion years ago in the _____ Bang . The faint afterglow we still detect is called the Cosmic Microwave _____ . The two simplest elements made first were hydrogen and _____ .
Big Background helium Small iron
