Answer: C M B (Cosmic Microwave Background)
Step-by-step explanation:
The theory of Big Bang suggests that matter of entire universe was converged at a single point thus, having infinite density. At time t = 0 (Big Bang), the universe started expanding.
This is no more a hypothesis but a theory because of the evidence provided by detection of C M B i.e. Cosmic microwave background. C M B is the remnant radiation of the Big Bang left over from about 400,000 years after the Big Bang. Before that the universe was opaque. C M B is present in all the directions and has an average temperature of 3 K. Thus, the detection of C M B led to proof of the Big Bang Theory.