antimatter
From CNN, July 22, 2017: Fermilab scientist Don Lincoln explains how the Deep Underground Neutrino Experiment, which had its groundbreaking ceremony on July 21, could answer important questions of the universe, including, “Where did the antimatter go?”.
From Wired, April 5, 2017: For some reason, more matter formed than antimatter just after the Big Bang, and physicists don’t know why. “It’s one of the very biggest mysteries in the universe,” says physicist Don Lincoln of Fermilab.
From Science, March 6, 2017: For more than a decade, multiple experiments have found an unexpected excess in the number of high-energy antielectrons, or positrons, in space. A team led by Fermilab’s Dan Hooper has shown that pulsars, not dark matter annihilation, can indeed produce most or all of the excess.