(covers information from several alternate timelines)
Antichronitons were subatomic particles, the antimatter equivalent of chronitons.
In 2373, upon learning that Kes had been uncontrollably traveling back in time from the moment of her death, The Doctor and Lieutenant Torres began a procedure to irradiate Kes with a modulated field of antichronitons to purge her system of the chroniton radiation to which she had been exposed in a future time period, and which, in combination with a biotemporal field to which the The Doctor had exposed her in the future, placed her in a state of biotemporal flux, causing her time jumps. Placing Kes in a bio-temporal chamber and initiating the antichroniton field, Kes experienced a series of more time jumps as far back as her own conception, at which point she returned to the present, and was brought back into temporal sync. (VOY: "Before and After")