THE collision of entire clusters of galaxies has helped set the firmest limit yet on the amount of antimatter in the universe.
In the early universe, the theory goes, matter and antimatter should have been created in equal amounts. However, our universe appears to be made of matter – though astronomers have never been able to rule out the possibility that there are antimatter galaxies out there.
In 2006, astronomers discovered the Bullet cluster – two clusters of galaxies that had collided, leaving a giant cloud of hot gas. If any fraction of this gas is a mixture of matter and…
