Motto: "O dark dark dark. They all go into the dark,
The vacant interstellar space, the vacant into the vacant"
(T. S. Elliot, in "East Coker", No. 2 of The Four Quartets)
Cosmology is the study of the universe as a whole, it is the science of the large-scale structure of the universe, its origin, and its evolution from the early times into the future. In this context universe means all that exists in a physical sense, not only the part of the universe that we can observe (Ellis 2007) using our telescopes and detectors on the ground and in space. The observable universe could certainly be a tiny fraction of the whole universe, even an infinitesimal fraction if the universe were infinite.
Cosmology today can be considered a branch of physics with a slight difference: we cannot experiment with the subject of our discussion, the universe, we can only observe it and model it.
The statement that "we are living in the era of precision cosmology" is certainly one of the most heard ones in the last 10 years in conferences, seminars and talks about the field. There is no doubt that within this period, modern cosmology has expanded from what Allan Sandage (1970) once described as "the search for two numbers", meaning the Hubble and the deceleration parameters. These and a few more numbers are conforming now a self-consistent set, derived from several different cosmological observations: high-redshift supernovae, fluctuations of the Cosmic Microwave Background (CMB) radiation, the large-scale structure of the universe, gravitational lensing, etc., but the emergent concordance cosmology provided by all these probes (sharing all beautiful hard-to-get cosmological data) is in a sense disappointing. We need to claim for the existence of gravitating non-baryonic dark matter of unknown nature, and furthermore, the universe today has to be dominated by an exotic dark energy, acting as a repulsive gravity. Some cosmologists take those theories as seriously as Ptolemy and colleagues took epicycles and deferents to reconcile the geocentric model with the early observations of planetary motion, or how physicists prior to the Michelson-Morley experiment considered the aether of undoubted certain existence. Since cosmology is not an experimental science, but an observational one, we must take this into account when we try to falsify our theories in the sense advocated by Karl Popper (1959): our requirement is just that our theories should be consistent with present and future observations. In contrast to what happens in experimental physics, astronomers cannot modify the object under study. They cannot control it in any way; they only can observe it many times, with different exposure times or at different frequencies or observe many objects of the same type (Kolb 2007a). This fact inevitably conditions the way we do our research and plan our observations.