Things we don't understand-

"Over the past decade, physicists have come to appreciate, to an even greater degree than in Einstein's day, just how little they know about how the cosmos works. The latest observations indicate that 95 percent of the universe consists of stuff we don't understand:

Dark matter, which can only be detected through its gravitational effect, makes up about 25 percent.
Dark energy, a property of empty space that seems to be pushing galaxies farther apart at an increasing rate, accounts for the other 70 percent.

"In a sense, it's the ultimate Copernican revolution," says Sean Carroll, a physicist at the University of Chicago. "Not only are we not at the center of the universe — we're not even made of the same stuff as most of the universe is made of."