Question: what is dark energy?

Sam Geen answered on 24 Jun 2013:

Supernovae are exploding stars, and are really bright, so you can see them in distant galaxies. We think we know how bright they should be, so by finding out how bright they are when we look at them, we can find out how far away they are (brighter things get dimmer when they get further away). From this we can work out how fast the universe is expanding, because we can measure how big the universe was at different times. Remember that the further away things are, the longer it takes light to reach us from them, so seeing very distant things is like looking into the past.

What people found was that, rather than just expanding, the universe is actually expanding faster and faster all the time. We know how fast it should be expanding from measuring everything that’s in it, and how much energy it has, a bit like how gas expands from pressure. What we found was that we couldn’t explain where most of the energy came from that was making it expand faster and faster like this. So we called this “dark energy” – in other words, energy that we can’t see but we think must be there, unless there’s something wrong with our measurements, or unless there’s something else weird going on.