Modern cosmology is the scientific study of the origin, development, and large-scale structure of the Universe . Before the 20th century, there was little hope or expectation of being able to answer such questions scientifically. Perhaps the most important milestones in cosmology were Galileo and Newton's developments of astronomical telescopes, Einstein's relativity theories, Hubble's discovery of the expansion of the Universe , and the Hubble Space Telescope. These conditions are known as the cosmological principle . To help you visualise this, imagine many dots painted on a balloon, which is then blown up.
Dots close together separate slower than dots further apart.
In the Big Bang Theory , the observable universe began with an instantaneously expanding point, roughly 15 billion years ago. Since then, the universe has continued to expand, gradually increasing the distance between our Galaxy and external galaxies. On the vast energy, distance, and time scales that govern the evolution of the universe , gravity dominates all other forces. Therefore, cosmology is very much concerned with the study of gravity and its interaction with energy and matter. If space-time is imagined as a rubber sheet, then massive objects such as stars and galaxies create deformations in it.