Dark matter
In 2006, a group of scientists claimed that they had found a way to find dark matter. Since dark matter is supposedly very different from normal matter, it is expected to act differently. The scientists observed two far-away galaxy clusters that had crashed into each other at high speed: normal matter would have been scattered nearby after the collision, while dark matter would not. By measuring gravity, they were able to detect what looked like two clouds of dark matter, with a cloud of normal matter (hot gas) in between them.