Dark energy

The BBC reports on the mystery of dark energy: no-one really knows where it comes from, yet it is known to be responsible for the ongoing acceleration of our universe's expansion. It seems likely to arise at the sub-atomic level, yet its effects are felt on the very largest scale. And these effects have been conclusively observed, experimentally, yet theoreticians are mostly still scratching their heads.

Then again, I guess if scientists did work out what dark energy was, they would have to rename it!