We know that gravity exists, and we know how it works. But we don't know why it exists.
And, the universe is getting bigger and it's moving faster. But no one knows why. And, that seems to be the opposite of gravity.
But they're working on this puzzle in Texas, in the Davis Mountains, at the McDonald Observatory ... the Hobbly-Eberly Telescope Dark Energy Experiment (to its friends, HETDEX).
"HETDEX is a project designed to understand the evolutionary history of dark energy."
What, you may ask, is dark energy? "We know very little about it beyond its existence." But it does make up 70% of all of the energy in the universe.
Einstein first came up with the ides of dark energy. Apparently it is a force that counteracts gravity.
http://www.nytimes.com/2011/06/19/us/19ttobservatory.html?ref=science
http://www.as.utexas.edu/hetdex/
