Antimatter
Each known particle has an antiparticle with which it shares all the same properties, except that some of the quantum numbers have the opposite sign. Electrically neutral elementary bosons (photons, gluons, Z bosons, neutral Higgs bosons) are their own antiparticles. The only known elementary fermions that are electrically neutral are neutrinos, and it is not known whether they are their own antiparticles (which would make them examples of what is called Majorana fermions) or not (Dirac fermions). Very-high-precision experiments are set up trying to find this out, by looking for something called "neutrinoless double decay" of certain radioactive atoms.
There is a popular statement saying that when matter and antimatter are brought into contact, they will "annihilate into pure energy." The annihilation part is true, but "pure" energy is misleading. In fact, a particle and its antiparticle can annihilate to other particles of lower total mass (because of conservation of energy). If the final state particles are much lighter than the annihilating particles, or even massless (photons!), much of the mass energy of the annihilating particles is converted into movement energy. If there are many annihilating pairs present, this means that much heat is produced, or colloquially: energy.
This is not so fundamentally different from what happens in a nuclear bomb: a lot of mass energy gets converted into movement energy, which means a huge increase in temperature, resulting in a scorching fireball and a gigantic pressure wave, which create the terrible destructive potential of the bomb. In the case of antimatter annihilation, the conversion from mass to heat could be much more complete than in nuclear fission. So even a small quantity of antimatter, say one gram, coming into contact with ordinary matter might free enough heat to produce an explosion like a nuclear bomb.
Should we be worrying about military applications of antimatter then? Well, antimatter is being produced all the time at CERN's Large Hadron Collider and other accelerators, but to produce anywhere near a gram using current technologies would probably require centuries1 of running the LHC, at huge electricity consumption. Moreover, one would have to come up with a way to efficiently slow down, capture, and store the antiparticles zooming away from the collision point in all directions at nearly the speed of light. This is nearly impossible. All in all, it's safe to say that there is no danger of antimatter bombs becoming a reality in the foreseeable future. But all that aside: CERN is dedicated to peace, which is all the more important given that conventional and nuclear weapons are very much a reality all over the world.
1 I haven't calculated it. If you do, please let me know what you find!