Cosmology can be a hard discipline to have a handle on, because it is a field of study inside physics that touches on a number of other areas. (Although, in fact, these days fairly much all areas of study within physics touch on many other areas.) What is cosmology? What do the people studying it (known as cosmologists) actually do? What evidence is there to aid their work?
Cosmology is actually the discipline of science that research the origin and eventual fate of the universe. It is most strongly related to the specific areas of astrophysics and astronomy, though the last century also has brought cosmology closely in line with important insights from particle physics.
The study of cosmology is probably one of the oldest types of speculative inquiry into nature and it started at some point in history when an ancient human being looked toward the heavens, asked questions including the following:
What is happening in the night sky?
How did we come to be here?
What are those shiny things in the sky?
Are we alone in the universe?
Physical cosmology, as a department of astronomy, is the analysis of the biggest scale structures and dynamics of the world and is focused on fundamental questions about its evolution and formation. For most of human history, it had been a branch of religion and metaphysics. Cosmology like a science originated with the Copernican principle, which means that celestial bodies obey the same physical laws to people on Earth and Newtonian mechanics, which first permitted us to know those laws.
Physical cosmology, because it is now understood, started with the 20th century progression of better astronomical observations of very distant objects and Albert Einstein's general theory of relativity. These advances created it possible to speculate in regards to the origin of the universe and permitted scientists to establish the Big Bang Theory as the top cosmological model. A few researchers still advocate a few of alternative cosmologies; however, cosmologists usually agree that the Big Bang theory best describes observations.