Is some of the universe anti-matter?
What evidence is there that the universe is primarily made of matter as opposed to antimatter? How would we be able to tell if roughly half the galaxies out there were primarily antimatter?
We take this question to Tamela Maciel, postgraduate astrophysicist at Cambridge University....
Tamela - If astronomers caught sight of a distant and isolated galaxy composed entirely of antimatter, they'd have trouble spotting the difference between it and a normal matter galaxy. That's because apart from having an opposite electric charge and magnetic moment, an antimatter particle has the same mass and internal structure as its matter counterpart. So light emitted from an excited antihydrogen would look exactly the same as from a normal hydrogen atom. And a galaxy of antimatter would have the same mass, and therefore the same gravitational force, as a matter galaxy. Antimatter is exotic neither because it is very different from normal matter nor because it follows different physical rules, but simply because it is very rare in our present-day universe.
Very high energies are required in order to produce antimatter. On earth, CERN's Large Hadron Collider routinely creates antimatter as the natural byproduct of high-energy particle collisions. The challenge is storing this antimatter for any length of time - every time antimatter collides with normal matter both are annihilated in a flash of light. And this happens a lot in our matter-dominated world. Nevertheless a few years ago physicists at CERN were able to study a newly-minted antihydrogen atom in isolation for a full 17 minutes before it eventually disappeared. In space, where much more energetic collisions occur, antimatter is also regularly detected. But usually only in small, discrete quantities arising from well-understood particle interactions.
So how do we know there aren't any antimatter galaxies lurking out there in the universe? The flash of gamma ray light that results from matter-antimatter annihilation is an excellent antimatter detector. If there are galaxies made up entirely of antimatter, then on the edges of these galaxies where the antimatter meets normal matter we would expect to see lots of gamma rays from annihilation. But despite careful observation from the likes of ESA's INTEGRAL or NASA's Fermi Gamma-ray Space Telescope, this tell-tale antimatter signature has not been observed, so astronomers believe that antimatter galaxies and clusters of galaxies do not exist.
But perhaps all the antimatter galaxies are sitting away by themselves in some distant corner of the universe where normal matter is rare. This possible separation of matter and antimatter into distinct corners of the universe must have happened immediately after the birth of the universe and before the period of rapid expansion known as inflation. But even in the high-energy soup of the very early universe, none of our current particle physics theories can provide a mechanism allowing antimatter and matter to separate spatially. Remember that in many ways, antimatter behaves just the same as normal matter and follows the same physical laws. So at the moment, it seems impossible that antimatter could have clumped and separated away from matter at any point during the history of the universe.
The biggest question in this discussion is why is there so much matter? Equal amounts of matter and antimatter should have been produced from the aftermath of the Big Bang. But with equal amounts, all the matter and antimatter should have annihilated long ago leaving only light. Instead there must have been a slight imbalance in the amount of matter produced compared to antimatter which meant that over time the annihilation process gradually left only the excess matter which forms our universe today. Despite a few potential theories, we still don't know exactly why the imbalance exists and this enigma continues to be a big topic of research.