[simonh]

You would think that the gravitational attraction of all the matter in the universe would slow down its expansion, at least a little. However we observe that the rate of expansion of the universe seems to be accelerating over time, and it seems like something must be causing that. The cause is called dark energy, but you’re right, we don’t really know what it is.

It’s analogous to the situation with dark matter. Again we hypothesise some additional factor that causes behaviour we wouldn't otherwise expect. We attach the terms energy and matter to them, because those are the things that generally cause the kind of effects we observe, but they’re placeholder terms and it’s always possible the observations or our theories are incorrect or incomplete.