3 main theories that are believed to be the origin of the universe are the big-bang theory ,steady state theory and the pulsating theory.
Before the big bang, all matter and energy in universe was compressed into a speck(a singularity)smaller than an atom. in the big-bang theory there was an explosive expansion happened. it was hardly believed that the density of a matter decreases over time and it was hardly believed also that the universe has been expanding ever since.
Meanwhile the steady state theory explains that the universe has always existed and will always exist and the universe is expanding throughout time after time, in this theory it tells us that the density of the universe does not change. Hydrogen gas bubbles up into the universe so that the density doesn’t change.
Lastly the pulsating theory, it was made to Believed that this theory tells us that the universe is supposed to be expanding and contracting alternately.
This Theories were made out of human curiosity, an hard evidence of human expedition as decades run through time.