Every particle in your body has a mirror twin. The electron has the positron; the proton has the antiproton. Same mass, opposite charge — and when a particle meets its anti-twin, both vanish in a flash of pure energy. The early universe should be full of these mirror particles. It is almost entirely empty of them, and nobody is certain why.

A prediction that came true backwards

In 1928 Paul Dirac wrote down an equation marrying quantum mechanics to relativity, and it spat out something awkward: solutions describing a particle just like the electron but with the opposite charge. Rather than wave the result away, Dirac took it seriously. Four years later Carl Anderson, sifting through cosmic-ray tracks, found exactly that particle — the positron. Antimatter was real, predicted by pure mathematics before anyone laid eyes on it.

It isn't science fiction, either. Hospitals use it daily: the "P" in a PET scan stands for positron emission. Even a banana, rich in potassium-40, flicks out a positron now and then.

The annihilation, and the exchange rate

When matter and antimatter touch, their entire mass converts straight to energy — the most complete conversion physics permits, and a vivid demonstration that mass is just frozen energy. Gram for gram, nothing else comes close. Which is exactly what makes the cosmic accounting so strange.

The missing mirror

Run the Big Bang forward and the simplest expectation is that it made matter and antimatter in equal amounts. Had it done so precisely, they would have annihilated each other completely, leaving a universe of light and nothing else — no stars, no planets, no readers.

Instead there was a faint imbalance. For roughly every billion antimatter particles, there were about a billion and one particles of matter. The antimatter found partners and annihilated; that one-in-a-billion surplus is everything that exists today. We are the leftovers.

Why the imbalance? The hunt for an answer is called baryogenesis, and in 1967 Andrei Sakharov laid out three conditions any explanation must satisfy:

1. Some process must be able to change the net amount of matter.
2. Nature must treat matter and antimatter slightly differently — a symmetry called CP has to break.
3. It must happen out of equilibrium, while the young universe was rapidly cooling.

We have actually caught nature in the act of condition 2: certain particles really do decay a touch differently from their antiparticles. But the effect is far too small to account for the surplus we owe our existence to. The Standard Model, for all its triumphs, cannot close the books.

So one of the most basic facts about reality — that there is a reality, made of matter — remains an open case. The mirror came up short, and we are what was left standing.