When we look out into space, we can see, on average, that the billions of galaxies in our surroundings are constantly receding away. The Universe, as it were, expands with every passing second.
Now imagine rewinding the situation to the earliest moment in cosmic history — by extrapolation, everything emanates from a infinitely small point, a “singularity”. This is the concept of the Big Bang: the idea that all that surrounds us — every particle, every star, every galaxy — once originated from an infinitely small and infinitely dense singularity, thrust into expansion in a single cataclysmic event. Quite what prompted the onset of this catastrophic process remains a mystery to this day.
The tremendous release of energy during the Big Bang can be traced all the way to the present day in the form of a faint afterglow, the Cosmic Microwave Background (CMB). In fact, this relic radiation pervades all space, and is manifest in our day-to-day lives as the static we see on television. Aside from being a slight hindrance to our TV viewing, the CMB carries information about the Universe only a few thousand years after its birth — in some sense, acting as a photograph of the infant Universe.