Big Bang theory may be considered “the leading explanation”, but the word ‘leading’ suggests that the evidence is not conclusive. It’s interesting therefore that you place quote marks around “know”. Why do that? Perhaps it’s not so clear after all.

You says that “astronomers can see at the leftover radiation from the Big Bang through a phenomenon known as the cosmic microwave background”. But the original connection back in the 60s between the Big Bang and the CMBR was based on some dodgy science.