Question: What is dark energy?

Richard Fielder answered on 1 Jul 2020:

If you can answer that, there’s a Nobel prize in it for you! Dark energy is essentially a placeholder name for something we can see must exist, but which we don’t really understand any more than that. It comes from the discovery that the expansion of the universe appears to be accelerating. According to older theories, the universe is expanding following the Big Bang, but it should be slowing down as gravity tries to pull everything back together. But when we started making more detailed observations to see how quickly it was slowing down, it turned out that it’s actually speeding up instead. Since you can’t accelerate an object without adding energy to it, that means there must be some energy we hadn’t noticed before giving it that push. We call it dark energy basically because we have to call it something while we try to figure out exactly what it is and why it’s there – it’s dark in the sense that we’re lacking knowledge (as in the Dark Ages).

The big problem is that it only seems to have any effect on very large scales – even between galaxies it’s tiny and you need to look at galactic clusters and bigger. That’s why we didn’t notice it before; the effects are just too small to see here on Earth. But because space is really big and there’s a huge amount of apparently empty space between galaxies and clusters, it seems that dark energy actually makes up the majority of energy in the universe. Unfortunately so far we don’t know much more than that it’s there and there seems to be a lot of it.