BIG BANG FACTS: NASA recently shared a refresher on the Big Bang — the event that gave rise to our universe. For those who haven’t thought about anything space related in some time, it occurred some 13.8 billion years ago.

It’s hard to fathom that, at the moment of the Big Bang, all of the energy in the universe — some of which would later become galaxies, stars, planets and human beings — was concentrated into a tiny point, smaller than the nucleus of an atom. And it’s not just matter that was born in the Big Bang. In the view of modern cosmologists, matter and space and time all arose when that microscopic point suddenly expanded violently and exponentially.

The Big Bang refers to a theory. How could it be otherwise? The current version of Big Bang theory — the one used most by modern cosmologists — is called the Lambda-CDM model. It postulates that our universe began at a specific instant, expanded to be flat (i.e. has zero curvature) and is made up of 5% baryons (i.e. the matter that makes up everything we see — galaxies, stars, planets, people), 27% cold dark matter (hence the “CDM” of the theory’s name) and 68% dark energy. But it is not without problems; as with all scientific theories, the Lambda-CDM model continues to evolve.

Now let’s pause a moment, so that we might draw a distinction between the appearance of all that energy in the Big Bang and its sudden expansion. In that sense, the Big Bang was not the event that caused our universe. Rather, it was the event that gave birth to the universe. Why is this distinction important? It’s important because, although science has been able to establish a history of the universe right back to when that tiny point suddenly created our entire cosmos, what preceded it, the reason for that tiny point of energy being there in the first place, is unknown, and may forever be unknowable.