The universe, for kids

Everything there is. It began about 13.8 billion years ago, it is still getting bigger, and the part of it we can see has an edge — not because the universe stops there, but because light has not had time to reach us from any further.

The size, in steps you can hold

Nobody can picture the universe. What you can picture is the ladder: the Earth is a ball you could walk round in a couple of years. The Moon is thirty Earths away. The Sun is four hundred Moons away. The nearest star is a quarter of a million Suns away. Our galaxy is a hundred thousand light-years across, and there are about two trillion galaxies. Each rung is a real measurement somebody made, standing on the rung below.

• Rung 1 — the size of the Earth — a stick and a shadow
• Rung 2 — the Moon — a curved shadow in an eclipse
• Rung 3 — the shape of the solar system — two pins and a string
• Rung 4 — the distance to the Sun — Venus crossing it, from two continents
• Rung 5 — the distance to the stars — the Earth's orbit as a baseline
• Rung 6 — other galaxies — stars that pulse in time with their brightness
• Rung 7 — the Universe — the colour of light from very far away

How it began

Everything was once packed into something unimaginably hot and dense, and it has been expanding and cooling ever since. That is not a guess — three separate pieces of evidence say so, and they were found by three groups of people who were not looking for them.

• How the universe began, and how anyone could know — the expanding sky, the leftover heat, and the recipe of the light elements

What it is made of

Ordinary matter — every star, planet, person and rock — is about five parts in a hundred. About a quarter is dark matter, which has gravity and nothing else we can detect. The rest, roughly two thirds, is dark energy, which is pushing the expansion faster and which nobody understands at all.

The Dark Matter Halopart of the galaxyNot observable

• What is left to explain — the honest list of what nobody has worked out yet

The strangest things in it

Stars so dense a teaspoonful would weigh as much as a mountain. A galaxy with a hole in the middle. Two galaxies mid-collision. A star that collapsed into a black hole and simply went out, with no explosion at all.

RX J1856.5-3754neutron starNot observableHoag's Objectring galaxyNot observableThe Antennae Galaxies (NGC 4038/4039)colliding galaxiesNot observableN6946-BH1failed supernovaNot observable3C 273quasarWith effortGW170817kilonovaNot observable

Then go outside

The universe is not somewhere else. You are in it, and about half of it is over your head right now.