This question was asked by Flora in London, UK.
This is a fantastic question and it’s one that has puzzled scientists for hundreds of years. It’s believed that it was first asked by the famous German astronomer Johannes Kepler in 1610, but there’s still no definite answer. We know that if the universe is infinitely large and infinitely old then the light from all the stars should make the night sky as bright as day, and yet we can see with our own eyes that this isn’t the case. This is known as Olbers’ Paradox. A paradox is a statement that seems to contradict itself and yet seems to be true, and this one is named after another German astronomer: Heinrich Olbers.
Part of the answer to this question is that our universe isn’t infinitely old after all; astronomers agree that it began somewhere between 15 billion and 12 billion years ago. Because light travels at a constant speed through the vacuum of space (299,792 kilometres per second) we can only see stars whose light has taken less than 12-15 billion years to travel to us. We can’t yet see stars that are further away.
This can be a tricky concept to understand, so let’s look at it another way. The distance that light travels in one year is called a lightyear, and it’s about 10 trillion kilometres. If a star is 10 lightyears away then it will take 10 years for its light to reach us; we’re seeing the star’s light as it was 10 years ago. If we look at a star that’s 100 lightyears away we’re seeing it as it was 100 years ago. If there are any stars that are more than 12-15 billion lightyears away, their light hasn’t reached us yet.
Equally, if aliens were looking at our star or our planet from 100 lightyears away, they’d be seeing it as it was in 1915. If they were viewing us from 65 million lightyears away they’d be watching the last dinosaurs! So if you think about it, when you look at the night sky, you’re looking back in time.
It’s also been suggested that the light from distant stars does reach us but that it’s too weak for us to see. Light is strongest at its source and becomes less bright the further away it travels – imagine shining a torch in a really big field at night. Although the light is definitely there, someone halfway across the field would only see it faintly and a person on the far side may not be able to see it at all.
Another reason may be that stars and even galaxies don’t last forever; eventually they will become dim and dark. We’ll see this happen to the galaxies and stars closest to us first (although not for an extremely long time!) because their light takes less time to travel to us than those far away. So even when the light from those distant stars does reach us, the stars and galaxies closest to us will have burned out and be dark so the night sky still won’t be full of light. We can never see all the light from all the stars at the same time.
(If you’re interested in looking more closely at distant stars, this article contains a link to an interactive copy of the above photo of the Andromeda galaxy).