Hint: it isn’t the Universe and there are 2 answers, 3 if you include the Universe.
We thought dark energy was a mystery, it isn’t if you revert to basics. Physicist’s figured out it was there the hard way, it’s simple when thermodynamics are applied.
If interested leave a reply, I think comments work, probably not and missed it from the 8 pages – close to it at least – to modify this site. New to this, barely knowing web stuff.