New Limit on Cosmic Antimatter
In the dramatic collision of two galaxy clusters in the Bullet Cluster, it wasn't the collision of a bullet and and an antibullet, or else the annihilation of matter and antimatter would have led to a strong gamma-ray signal that was not seen. This new result has been shown by
Prof. Gary Steigman in http://arXiv.org/abs/0808.1122, and reported on in New Scientist at http://space.newscientist.com/article/dn14650-galactic-supercrash-proves-rarity-of-antimatter.html
The Bullet Cluster is so called because it reveals the aftermath of a collision of two clusters, each composed of dark matter and ordinary gas. The dark matter clumps -- revealed by gravitational lensing -- passed through each other easily, while the gas clumps collided and were distorted into bow shocks.
