The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.
Cosmology is a broad discipline, seeking answers to the "big questions". It has connections to many sub-fields of physics and astrophysics, and it can be approached from a variety of different angles.
Cosmologists may be interested in cataloging the contents of the Universe, at least in broad categories, and determining how they are arranged. This facet of cosmology is closely related to astronomy. It includes such questions as:
- How much dark-matter  and dark-energy  are there in the Universe?
- What are the typical densities of baryonic matter and photons?
- To what extent is matter clumped hierarchically?
Another facet involves studying the very early conditions of the Universe, usually according to some variant of the big-bang  model. Informed by particle-physics, cosmologists deal with (among other inquiries):
- cosmological-inflation  and other methods for overcoming quandaries in our models regarding their initial conditions;
- baryogenesis , the formation of such particles as protons and neutrons; and
- Primordial nucleosynthesis , the process by which some baryons coalesced into various light elements.
One can also ask about the growth of the Universe. Aided with the machinery of general-relativity, cosmologists can study the large-scale, global evolution of spacetime. Along these lines, one can ask about
- The expansion history of the Universe - i.e. how the scale factor has evolved over time;
- The average geometry or curvature of the Universe, and to a certain extent its topology; or
- The ultimate fate of the Universe, such as whether it will expand forever or eventually re-collapse.
Related concepts include: