The international KTeV research team came to this conclusion after analyzing millions of kaon-decay events.
Nevertheless, says KTeV collaborator Edward Blucher (University of Chicago), "the only other place where matter and antimatter [are known to behave differently] is in these neutral kaon systems." At least that's how things stand at present; accelerators in California and Japan are currently generating even more ephemeral particles, called B mesons, in the hopes of shedding new light on this outstanding cosmic puzzle.