What is "pure energy" in matter-antimatter annihilation made of? I used to read the term "pure energy" in the context of matter-antimatter annihilation.  Is the "pure energy" spoken of photons? Is it some form of heat?  Some kind of particles with mass? 
Basically, what does "pure energy" in the context of matter-antimatter annihilation refer to? 
 A: At low energies of matter-antimatter collision, the end products can only be massless particles like photons -- hence annihilation is conventionally associated with the production of radiation/light.
At higher energies, the collision can have enough energy to generate pairs of massive particles like mesons and W bosons.
The LEP was an electron-positron collider and it has the record for producing the most massive particles from such an annihilation process (W+ and W- bosons).
A: If I ruled the world, I would ban the phrase "pure energy" in contexts like this. There's no such thing as pure energy!
When particles and antiparticles annihilate, the resulting energy can take many different forms -- one of the basic principles of quantum physics is that any process that's not forbidden (say, because of violation of some sort of conservation law) has some probability of happening. So when a proton and an antiproton annihilate, they can produce photons, or pairs of other particles and antiparticles, such as a neutrino-antineutrino pair, or a positron-electron pair.
Although all sorts of things are possible, by far the most common product of matter-antimatter annihilation is photons, especially if the collision occurs at low energy. One reason is that lower-mass particles are easier to create than high-mass particles, and nothing has less mass than a photon. (Other particles, particularly neutrinos, have so little mass that they are "effectively massless," but neutrinos are weakly interacting particles, which means that the probability of producing them is lower.)
A: In particle antiparticle annihilation the end products come from the table of elementary particles  created so  that  quantum numbers are conserved.
photons, two to conserve momentum in the cmsystem, pions and other mesons etc.
A: Energy is the ability of a system to perform work on another system. In other words, it's a property of a physical system. The annihilation of an electron and a positron performs work on the electromagnetic field by creating two photons. We usually do not talk about these processes in the same language that we use in classical mechanics, but there is absolutely nothing wrong with thinking about it in exactly the same way. We merely have to replace the names of the systems. In classical mechanics we may have been elevating a mass "above the floor" with a pulley by performing mechanical work on it. In the case of fields we are creating field excitations that are "above" the ground state.
A: All the three above answers basically are correct and saying almost the same thing, i.e. the end product of matter-antimatter collision is photon at low energies and massive particles such as mesons... at high energies. However,by thinking of the reverse action namely production of matter from a form of energy ( radiation/light) and the source of that unimaginable huge energy,simply using E=mc*c,supports the above mentioned answers firmly. AA 7 June 20011 at10pm
