The Nobel laureate Philip Warren Anderson once said, “It is only slightly overstating the case to say that physics is the study of symmetry.” Because physics is the study of the behavior of the universe, Anderson’s statement implies that the universe is, to some degree, symmetrical.

But what does that even mean?

Symmetry, as scientists use the term, describes something that remains unaffected by transformations, like changes in time, place, or orientation. For example, consider the act of throwing a ball in the air. Assuming you throw the ball the same way under the same conditions, does it matter if you throw that ball on a Wednesday versus a Friday? Nope. That’s a symmetry of time. The shift in time alone has no effect on the trajectory of the ball.

The symmetry principles of the universe allow us to study it. Because the laws of physics are the same everywhere in space and time, scientists are able to make conclusions about things unseen by observing what they can. But for all the wonders that symmetry affords us, asymmetry is what allows us to exist.

You see, just after the Big Bang, the universe was a primordial soup made of light. When light is at a high enough energy, it creates matter. But when matter is born, so is its twin: antimatter. Every time a particle of matter is created, an antiparticle is created. But an interesting thing happens when matter and antimatter meet: they annihilate each other.

Since we are made of matter, this means we shouldn’t exist.

The Big Bang should have produced equal amounts of matter and antimatter, resulting in a neutral universe without either. Instead, at the beginning of time when matter was popping in and out of existence at an unfathomable rate, an imperfection happened. And for every billion antiparticles, there was a billion and one particles. That tiny surplus of matter led to the universe as we know it. That tiny surplus of matter is you.

And yet no one knows why this happened. This asymmetry of matter and antimatter remains one of the greatest unsolved problems in physics.