Is anti-matter a viable source of energy? I was wondering whether it is possible to harness energy created by small anti-matter/matter particle annihilations. I known that when they meet, a huge flash of light is produced by high energy gamma-rays (is that right?), so is it possible to somehow harness that energy, or is the sheer amount of energy discharged rule out any way of safely taking advantage of it?      
 A: No not really. Antimatter annihilations are incredibly inefficient due to the fact that we require huge amounts of energy to create it  and more energy to store it. The energy output would be about $10^{-10}$ of the input. So right now it's not viable.  
(Antimatter production costs about $25 billion per gram of positrons)
A: There seem to be a lot of misconceptions around things like Antimatter and even Fuel Cells, and they all come down to one thing -
Antimatter is an energy store, not a source.
Our current method for producing antimatter is to use the output of high-energy particle collisions. By smashing atoms into eachother and waiting for their component quarks to recombine, you can sometimes get antimatter - sometimes. Even if you do produce antimatter, there's no guarantee that it won't immediately annihilate with some nearby element of normal-matter. This makes producing antimatter a complete crapshoot.
So, you'r running a massive particle accelerator at extremely high energies and gobbling down power to produce at most a couple anti-atoms. Even if annihilation were perfectly efficient (which for reasons I'll cover later it isn't) you're never, ever going to make an energy-profit doing this.
Another example of the same problem is using Fuel Cells in vehicles. They produce electricity from hydrogen and oxygen, and exhaust only water. This sounds great on paper, until you realize that it's really challenging to store hydrogen safely, and hydrogen, most often, has to be produced. There are a couple of ways to produce hydrogen. Let's mention some of them. When diluted mineral acids (such as hydrochloric or sulfuric acid)  react with some metals that act as reducing agents (such as zinc or aluminium), hydrogen is produced (2Al + 3H2SO4 → Al2(SO4)3 + 3H2). Water reacts with alkali metals and alkaline earth metals to produce hydrogen (2H2O + 2Na → 2NaOH + H2). Hydrogen can also be produced by water electrolysis. Electrolysis is a very inefficient and power-hungry process, so again we're spending energy to make energy, but paired with renewables like solar power for example, we can extract hydrogen from water, where it could also act like a form of "battery" to store solar power for a rainy day.
Yet another problem with antimatter is that it produces neutrinos. This means that no matter how perfectly efficient your solution your antimatter generation, you'll still lose around half of it to completely non-interacting and unusable particles.
So why do people talk about it?
As I said, it's a fantastic store of energy. I could hold the equivalent the worlds power output (About 4*10^20 joules) for an entire year in about 60 grams of antimatter. This would be perfect for a starship, because instead of the mass of a worlds worth of power plants I could power an entire interstellar voyage with a few coffee cups worth of fuel. That said, all that power would have to come from somewhere. It's a great way to bundle some energy up and take it with you, but by no means is it going to solve our energy problems.
