Gravity will ultimately determine the fate of the expansion, and gravity is dependent upon the mass of the universe; specifically, there is a critical density of mass in the universe of 10 - 29 g / cm3 (equivalent to a few hydrogen atoms in a phone booth) that determines what might happen. (science.howstuffworks.com)
Everything with mass in the universe theoretically creates them — you and me included — but only highly cataclysmic events, such as exploding stars, colliding black holes, or the Big Bang, can generate waves that are powerful enough for LIGO to detect. (businessinsider.com)
During the 496th Brookhaven Lecture, Lijuan Ruan explained how she uses electron - positron tomography from quark - antiquark annihilations to study chiral symmetry, a characteristic that «broke» to form 99 percent of the visible mass in the universe and is thought to be restored during ion collisions at the Relativistic Heavy Ion Collider. (bnl.gov)