Nearly 25 years after its discovery, the mystery at the core of dark energy persists.  Astronomers are no closer to understanding what’s behind this cosmic repulsive force that counteracts gravity and causes the cosmos to expand at an accelerating rate than when it was first discovered in 1998.  Guest Alexei Filippenko is a member of the Nobel Prize-winning team that detected dark energy via supernovae surveys. He gives us the inside scoop on how dark energy was detected; what it means for our existence and the prospects for unmasking this bizarre force of nature that makes up some 70 percent of the observable universe.