In 2007,
Astronomy published a special issue called “The 50 greatest mysteries of the universe.” Part of the reason we were able to call the article in the August 2013 issue “The 40 greatest mysteries of the universe” is that scientists are continually figuring out the solutions to former problems. That is, after all, what they get paid for. Here, we highlight some results that have come in since our 2007 publication.
Astronomy magazine subscribers can read the full article for free. Just make sure you're registered with the website.