Origins of The Universe
There are two main ideas of the origins of the universe, the 'steady-state theory' and the 'big bang theory'. The big bang theory is the most believed today.
The steady-state theory suggests that the universe is always existing but maintains the same average density which suggests that new matter is always being created to form new planets, galaxies, etc at the same rate that old ones become unobservable. A universe that goes by the steady-state theory has no beginning or end in time. There is some evidence that can support this idea. The evidence is redshift which is whereas objects move further away from us (galaxies) the light emitted by them shifts towards the red end of the spectrum. This supports the steady-state theory as it suggests that the universe is always expanding.
However, when cosmic microwave background radiation (CMBR) was discovered which is effectively the 'afterglow' of an explosion, for example, the big bang. This contradicts the steady-state theory, leading many people to disbelieve it.
Big bang theory:
The big bang theory is most believed today and has the most evidence which supports it. Both CMBR and redshift support this theory. The big bang theory suggests that the universe was created from an explosion of an incredibly hot and dense point that sent matter flying out creating planets and galaxies as we know it today. The big bang theory suggests that the universe is always expanding and so is supported by redshift as objects are moving away from us since the light emitted by them is shifting towards the red end of the spectrum. Since both CMBR and redshift support the big bang it is most widely believed today.