All science information is really considered to be theories because nothing can be proven completely true. For instance, the biggest theory is probably the Big Bang Theory. Many scientists have been studying that for years and some of them believe it and have some proof to support their claim whereas other scientists have proof to disprove the theory. It really just matters who you ask.
So, all of cosmology is really just theories that have some proof attached to it. The exploration of the universe is relatively new and it is probably the most difficult science to prove due to it being so far away and so foreign to everyone. Over time, more proof may be discovered in order to make the theories more believable.
With study of our universe and beyond, matters defying ordinary observation, we are dependent upon theories for discovering new facts. If scientific, theories make testable predictions which, if they turn out to be wrong, would mean that the theory must be discarded. With regard to the Big Bang, some of those predictions have been tested, some to very high precision, and turned out to be correct.
This is thanks to work such as Hubble's Law - distant objects are moving away from us; the cosmic microwave background - as the universe gets smaller its temperature rises; the abundance of helium - whereas the missing part of the proof of Big Bang - cosmic inflation, has been impossible to test. If inflation turns out to be not falsifiable, this would make it impossible to test using the scientific method. This part remains a theory yet to be proved.