Question: What is dark energy?

Stewart Martin-Haugh answered on 12 Nov 2018:

Good question! We know that the universe is expanding – all of it is expanding in all directions, like points on a balloon:

The universe is expanding starting from the Big Bang a few billion years ago. But this expansion is getting faster, which is weird. Gravity pulls things together and you would expect that to slow it down. So there is an additional force that’s pushing everything apart – that’s called Dark Energy. It seems to be everywhere around us, so it must be part of space-time.

No one knows what it is. Maybe someone in your generation of physicists will find out!