The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it talks about the universe as we know it starting with a small singularity, then inflating over the next 13.8 billion years to the cosmos that we know today.

Because current instruments don’t allow astronomers to peer back at the universe’s birth, much of what we understand about the Big Bang Theory comes from mathematical theory and models. Astronomers can, however, see the “echo” of the expansion through a phenomenon known as the cosmic microwave background.

The phrase “Big Bang Theory” has been popular among astrophysicists for decades, but it hit the mainstream in 2007 when a comedy show with the same name premiered on CBS.