In the first moments after the Big Bang, matter and antimatter should have been created in equal amounts. But today, everything around us is ordinary matter, and antimatter is almost nowhere to be found. Where did it all go? The standard answer: a tiny imbalance in favor of matter wiped out almost all antimatter. A new model offers a different scenario: antimatter didn't vanish completely: it clumped into vast structures, entire galaxies made of antimatter.

The model builds on the idea that in the early hot universe, there could have been regions where antimatter dominated. These regions collapsed under their own gravity and formed stars and galaxies, just like ordinary matter. The authors argue that such antigalaxies should leave observable traces. One is anomalies in cosmic gamma-ray emission: when antimatter meets ordinary matter, characteristic flashes are produced. Another is the peculiar distribution of galaxies in space, which is hard to explain without antimatter.

Testing the model is a job for future telescopes, but already it explains several puzzling observations. For instance, some gamma-ray sources lack obvious conventional explanations. Moreover, antimatter could partially account for dark matter — the mysterious substance that holds galaxies together. If at least part of dark matter is antimatter in a hidden form, that changes our worldview.

For now, it's a bold hypothesis, but it reminds us that the universe might be trickier than we tend to think. Perhaps somewhere out there, billions of light-years away, there are stars whose light is indistinguishable from ours, yet they are made of mirror matter. And if we ever encounter such a galaxy, the consequences would be monumental — literally.