Short answers, pulled from the story.
Physical cosmology is a branch of physics that models the universe based on the laws of physics. A cosmological model gives a mathematical description of the largest-scale structures and dynamics of the universe, allowing study of its origin, structure, evolution, and ultimate fate.
Physical cosmology as it is now understood began in 1915 with Albert Einstein's general theory of relativity. Major observational discoveries followed in the 1920s, including Edwin Hubble's finding of galaxies beyond the Milky Way and evidence that the universe is expanding.
The Belgian Roman Catholic priest Georges Lemaitre proposed in 1927 that the universe began with the explosion of a primeval atom, later called the Big Bang. The theory was advocated and developed by George Gamow.
The standard cosmological model is the Lambda-CDM model, also written as Lambda cold dark matter. It includes a cosmological constant denoted by Lambda and associated with dark energy, together with cold dark matter.
About 23% of the mass of the universe consists of non-baryonic dark matter, and only 4% is visible baryonic matter. If the universe is flat, a further 73% of its energy density is dark energy.
The cosmic microwave background was first observed in 1965 by Arno Penzias and Robert Woodrow Wilson. It has a perfect thermal black-body spectrum, a temperature of 2.7 kelvins today, and is isotropic to one part in 100,000.
Observations suggest the universe began around 13.8 billion years ago. According to the Lambda-CDM model, it will continue expanding forever.