Cosmology is a branch of astronomy which studies the structure, origins, and space-time relationships of the universe. A cosmologist is one who studies this particular branch of science. Cosmology also refers to a branch of metaphysics that deals with the nature and origin of the universe.
According to Kathryn Janeway, Federation cosmologists had a theory, following the discovery of the Omega particle in the 23rd century, that revealed that "the molecule once existed in nature for an infinitesimal period of time at the exact moment of the Big Bang. Some claimed Omega was the primal source of energy for the explosion that began our universe." Seven of Nine, however, dismissed it as "a creation myth like any other." (VOY: "The Omega Directive")
Crewman Mortimer Harren studied this field. He held five advanced degrees in theoretical cosmology and needed one year of hands-on experience in order to qualify for the Institute of Cosmology on Orion I, so he joined the crew of the USS Voyager. He had worked on Schlezholt's Theory of Multiple Big Bangs and studies in dark matter proto-comets. (VOY: "Good Shepherd")