If you're into science you might find this article by Michael Brooks at New Scientist interesting. Brooks lists thirteen mysteries that have (mostly) cosmologists and physicists baffled. To give you a sample here's #9:
It's one of the most famous, and most embarrassing, problems in physics. In 1998, astronomers discovered that the universe is expanding at ever faster speeds. It's an effect still searching for a cause - until then, everyone thought the universe's expansion was slowing down after the big bang. "Theorists are still floundering around, looking for a sensible explanation," says cosmologist Katherine Freese of the University of Michigan, Ann Arbor. "We're all hoping that upcoming observations of supernovae, of clusters of galaxies and so on will give us more clues."
One suggestion is that some property of empty space is responsible - cosmologists call it dark energy. But all attempts to pin it down have fallen woefully short.
It's also possible that Einstein's theory of general relativity may need to be tweaked when applied to the very largest scales of the universe. "The field is still wide open," Freese says.
I suspect that cosmologists are much more confident that dark energy is the culprit behind the accelerating expansion of the universe than what this article suggests. Nevertheless, the most fascinating thing about dark energy, if it exists, is this:
Scientists have determined that the amount of dark energy present in the universe cannot vary from the actual value by more than one part in 10(120). That's a one with 120 zeros after it. If it did deviate from its actual value by more than this amount life would not be able to exist in the universe that would result. That is an incomprehensibly precise setting. It's the equivalent of the mass of a billionth of a trillionth of a trillionth of the mass of a single electron.
What an amazing thing it is that this dark energy is calibrated to just the right value to allow life to survive. What an extraordinary amount of blind faith it takes to think that it's just a lucky coincidence, especially when there are a couple dozen other forces and parameters of the cosmos which must also be set with similar precision in order for the universe to be life-sustaining.
The biggest mystery about dark energy, it seems to me, is not what it is but rather how it came to be so exquisitely calibrated. Or maybe that's only a mystery for materialists.