The Universe — Galaxies and Space

What Is the Universe?

The universe is everything that exists — all matter, energy, space, and time. It contains all the stars, galaxies, planets, black holes, dust, gas, and even the laws of physics themselves. As far as we know, there is nothing "outside" the universe, because the universe is, by definition, all there is.

The observable universe — the part we can see from Earth — has a diameter of about 93 billion light-years. A light-year is the distance light travels in one year: about 9.46 trillion kilometres. These numbers are so vast they are almost impossible to comprehend in everyday terms.

What Is the Big Bang?

The Big Bang is the leading scientific explanation for the origin of the universe. About 13.8 billion years ago, all the matter and energy in the universe was compressed into an incredibly hot, dense point (called a singularity). Then, in a fraction of a second, it began expanding — rapidly inflating in what we call the Big Bang.

It is important to understand that the Big Bang was not an explosion in space — it was an expansion of space itself. The universe did not burst outward from a central point. Instead, everywhere in the universe simultaneously became less dense as space stretched in all directions.

Evidence for the Big Bang includes the cosmic microwave background radiation (the faint glow left over from the early universe, detected in all directions), the observed expansion of the universe, and the abundance of hydrogen and helium in the universe.

What Is a Galaxy?

A galaxy is a massive collection of stars, gas, dust, and dark matter, all held together by gravity. Our galaxy is called the Milky Way — it contains between 100 billion and 400 billion stars. From Earth, on a dark night, you can see a faint band of light across the sky — this is the glow of billions of stars in our own galaxy, seen edge-on.

Galaxies come in different shapes:

• Spiral galaxies — flat discs with spiral arms, like the Milky Way and the Andromeda galaxy.
• Elliptical galaxies — oval or spherical, typically older and containing less gas and dust.
• Irregular galaxies — no defined shape, often the result of galactic collisions.

The Milky Way is part of a small group of galaxies called the Local Group. The Local Group is part of a larger cluster called the Virgo Supercluster, which is itself part of an even larger structure called Laniakea — a supercluster containing about 100,000 galaxies.

Black Holes

A black hole is a region of space where gravity is so strong that nothing — not even light — can escape. Black holes form when massive stars collapse under their own gravity at the end of their lives. The boundary around a black hole from which nothing can escape is called the event horizon.

Black holes come in different sizes. Stellar black holes (formed from collapsed stars) can be 5–100 times the mass of the Sun. Supermassive black holes, found at the centre of most large galaxies, can be millions or even billions of times the mass of the Sun. The Milky Way's central black hole, Sagittarius A*, is about 4 million solar masses.

In 2019, the Event Horizon Telescope produced the first direct image of a black hole — the supermassive black hole at the centre of galaxy M87, 55 million light-years from Earth.

Dark Matter and Dark Energy

Here is one of the most remarkable facts in modern science: the ordinary matter we can see — stars, planets, gas, dust — makes up only about 5% of the universe. The rest is mysterious.

• Dark matter (27%) — an invisible form of matter that does not emit or absorb light. We know it exists because of its gravitational effect on galaxies (galaxies spin faster than they should if only visible matter were present). Its exact nature is unknown.
• Dark energy (68%) — an even more mysterious force that is causing the expansion of the universe to accelerate. It was only discovered in 1998 and remains one of physics' greatest unsolved mysteries.