First theorized to exist back in the 1940s, the leftover glow from the Big Bang — also known as the cosmic microwave background, or CMB — was confirmed to exist back in the mid-1960s. Although many have questioned its interpretation and ultimate origin, its detailed properties not only confirm its emergence from a hot Big Bang, but provide many other cosmic insights. Yet still, even today, one huge cosmic mystery looms large: the Hubble tension. Here’s what we know about the CMB, plus what still remains to be discovered about our Universe.