The frequency at which it happens is related to a small but significant difference between the mass of the particle and its antiparticle.

ECONOMIST: Cosmology and particle physics

To assess the contribution from melting ice sheets, scientists try to measure "mass balance, " which is the difference between the annual snow that falls on the permanent ice sheets each year, and the total mass of ice that melts or breaks off the sheets.

WSJ: Polar Ice Sheets Melt at Faster Pace

On average, men have 40% more fat-free mass than women, which is similar to the difference in gorillas, a species in which males unquestionably compete with other males for exclusive sexual access to females.

ECONOMIST: Masculine traits

There also is a significant difference between what physicists believe should be the total amount of mass in the universe and what they have observed so far.

ENGADGET: AMS detects excess of positrons, could suggest existence of dark matter