You cherry-picked a definition.
Miriam-Webster
Theory: (n.) a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena (the wave theory of light)
and
The analysis of facts in their relation to one another.
Fact: Electromagnetic radiation has been detected from outer space.
Fact: This radiation has levels of energy, timing, and location that are heavily correspondant with those which scientists, through theoretical calculations, have found would have originated in a sort of fiery cataclysm.
Fact: From this, we can derive that, perhaps, (it's just a theory) the universe and its corresponding spacetime originated in a fiery cataclysm.
Theory: Big Bang.