Antimatter Storage How would it be possible to store a large amount of antimatter (say, 1000000 kg) for a long amount of time (relatively speaking, like around 100 years, or long enough to travel to other systems in interstellar space)? I know that this would be impossible with our current technology but is it be theoretically possible? 
 A: It is not possible to store this mass of antiprotons , if you see how they can be stored for laboratory uses:

Several hundred antiprotons of 2.1 GeV/c were produced by protons from the PS accelerator and were kept circulating in a machine called ICE (Intial Cooling Experiment) for a period of 85 hours i.e. about 300, 000 seconds (3 × 105). The previous best experimental measurement of antiproton lifetime, acquired during bubble chamber experiments, was about 10-4 second, i.e. a ten-thousandth of a second. 

It is an old link, but there are so many orders of magnitude difference to  your requirement that I did not bother to look further.
Antihydrogen would seem more promising but is  way beyond your needs.

Physicists with the international ALPHA Collaboration at CERN in Geneva have succeeded in storing a total of 309 antihydrogen atoms, some for as long as 1,000 seconds (almost 17 minutes) or even longer -- more than enough time to perform meaningful scientific experiments on confined anti-atoms.

In addition the magnets and Penning traps are such huge instruments , that it would be futile to fit them in a space ship.
So the answer is , no, it is not possible to store antimatter in the large numbers needed for your ideas, even in theory, as far as technology has progressed up to now, and can be projected in the future. 
A: Theoretically it is entirely possible. Just place a block of anti-tungsten in a perfect vacuum and hold it in place using magnetic fields (it is paramagnetic)
Is this actually doable according to known physics? There are a few problems: (1) evaporation from the antimatter or container walls causing drifting atoms, and (2) making anti-tungsten. I will ignore (2) which is of course a magnificent can of worms and instead deal with (1).
The vapour pressure of tungsten is $\approx 10^{7.933-45087/ T }$ atmospheres. Already at 300K this produces a pressure so low that one should not expect any loose atoms, but since we get to work in theory-land we can of course make $T$ as low as we want. Same thing for the container walls. We can add turbomolecular pumps to maintain low pressure. 
Still, in real 3K cryogenic vacuum systems 6.7 fPa has been achieved, corresponding to 100 particles per cubic centimetre (indeed, just having the system open to interstellar space would reduce this density by a factor of $\approx 100$). If we assume hydrogen molecules their RMS speed is $v_{rms}=\sqrt{\frac{3RT}{m}}=192.6778$ m/s. So the surface of the anti-tungsten will be hit by about 19200 molecules per square meter per second. That corresponds to $1.1551\times 10^{-5}$ W/m$^2$. In terms of heating this can be balanced by keeping the chamber walls somewhat cooler so the antimatter radiates away the heat. 
But the effect of the molecular impacts will likely cause trouble. When a proton-antiproton annihilation occurs there are on average 5.3 pions released with average energy 350 MeV, and since they are strongly interacting they can deposit up to 2 GeV in the host nucleus - enough to scatter it (oops, maybe should have used anti-iron instead, not that it would have helped much). Since these are surface nuclei we should expect at least half of the disintegration products to fly off at relativistic speeds (which makes capturing them with the magnetic field hard). We hence end up with on the order of $10^4$ antimatter nuclear fragments per second and square meter heading off to cause havoc. Since they are often heavier nuclei they also pack a bigger punch than the initial hydrogen when they hit a wall. Plus, this reaction may dislodge other antimatter atoms. 
Can this be fixed? A large chamber area can spread out the damage, but since we are working in a high vacuum there is no way of sucking out the fragments. We can cover the walls with turbomolecular pumps, but they will only act on matter fragments (the antimatter ones will annihilate and release more fragments). The pumps may be able to give them enough momentum to move in some safer direction for disposal but it looks tricky.
Overall, for a sufficiently low pressure (remember, the above value was 100 times above interstellar vacuum) and a large chamber with properly shaped walls I suspect the multiplication rate can be kept low enough that there is just a background loss of a few thousand atoms per second per square meter, enough to keep the antimatter essentially forever. Very clever designs of the magnetic field cradle may help. But it looks like it requires some serious engineering calculations and ingenuity. And any puff of gas accidentally released will quickly start a chain reaction that ends with hot ambiplasma.
This is engineering difficulty/impossibility (not practical as far as we can see, but could perhaps be solved with sufficiently devious tricks and overengineering). One should not confuse it with theoretical impossibility (not allowed by the laws of physics as we know them). A proper theoretical impossibility proof would require showing that e.g. the multiplication rate of fragments is always big, or that there is some in principle reason why antimatter cannot be cooled or made dense enough to crystallise.
