Which of the following is an unknown form of energy?

In the same way that black holes were known and scientifically accepted as existing, before our ever seeing one, dark energy is considered (some might plausibility argue - given recent scientific news- 'known') to be the anti-gravitational energy, which explains the above expansion.

Wow, this is the first time I have read something about dark energy that I actually understood!

Questions like this really make me wonder what it's all about. I suppose, in the end, we'll never really know. Space is so fascinating!

Yeah I have "dark energy", but mine is known and has a name.....Laziness!!!!! 😉

Gravity to this day is NOT Completely Understood.

Spooky, it's hard to explain something when scientists don't know that much about it.