The ingredients of the universe are baryonic matter, dark matter, dark energy, radiation and neutrinos. We focus on the two main ingredients which is matter (both baryonic and dark) and dark energy.
The Big Bang gives the universe an initial kick to begin expanding. How do the different ingredients of the universe affect this expansion? What we are looking for is the equation of motion of the universe. The Friedmann equation is the equation that describes this. It tells us how the universe's expansion is affect by varying amounts of matter and dark energy.
Matter wants to oppose the expansion of the universe due to gravity. In the same way that a ball thrown in the air on earth wants to oppose the throw and come back due to the earth's gravitational pull. Dark energy aids in the expansion anad at some point in the universe leads to an accelerated expansion. It would be as if the thrown ball would take off in the direction of the throw and never return to the earth.
The Friedmann equation tells us how the universe will expand given a certain amount of matter and dark energy. Will the universe collapse back on itself because of all the matter in the universe? Will the universe expand forever and ever? We gain an important clue about the universe by studying how much it has expanded from the Big Bang until now. We learn how much matter and how much dark energy are in the universe. We can also make predictions about the fate of the universe given these proportions.
This motion of the universe is the large scale behavior. In this scenario, you treat everything as a homogeneous substance. The next natural extension to that question is surrounding the fine details of that homogeneous substance. How does matter behave on smaller scales that lead to the galaxies and stars that we see today? These are the inhomogeneities of the universe that we observe today. How and when did this come about in the history of the universe?
This in an interesting question that the 'structure formation' community studies in detail.
image credits : science progress website, chandra xray observatory
