What material is it? Its something known as antimatter.
Antimatter is the opposite of regular matter because although it is made up of particles and atoms just like regular matter, it has opposite electrical charge and has negative protons called anti protons.
Whenever antimatter touches regular matter, the two instantly annihilate each other.The pure energy released is why antimatter is so useful and expensive.
To put it into perspective, if you dropped a raisin-sized amount of antimatter on the ground, it will create an explosion bigger than the Hiroshima and Nagasaki combined.Also if you made an antimatter bullet, when fired, the bullet would be able to destroy an entire house.
Antimatter can also be used as fuel source for interstellar travel, and has the potential to allow us to reach the nearest star to earth on around two years. But why haven't we done any of these things? That's because antimatter is incredibly rare, difficult to produce and expensive.
According to NASA paper, antimatter cost around $25 billion per gram.Another NASA paper from 1999, however, says that a gram of antimatter cost up to 62.5 trillion per gram. The only way to acquire antimatter is to acquire it artificially, which is incredibly difficult.
The CERN research facility in Switzerland is one facility that produces anti protons.It can produce 10 million anti protons a minute, which is actually close to nothing. At its current rate of production, it would take 100 billion years to create just 1 gram of antimatter.
Finding antimatter is a problem, but storing antimatter is another huge problem.you have to suspend the material so that it doesn't come in contact with anything. So far, antimatter has been stored for 17 minutes max before its completely annihilated because of contact with regular matter.