An Anti-matter hamster
We are all made of matter. As Einstein once said, matter is energy and energy is matter and can we all go to the pub now I'm bored?
Now our matter is made up of atoms - tiny little popper beads that you can stick together to make up bigger lumps called molecules, which can themselves be lumped together to make up...well...us.
Each atom is made up of a centre bit made of any number of protons, and a whizzy around the centre bit made up of any number of electrons. The amount of protons and electrons determines what an atom is (i.e. Hydrogen, Helium, Aluminium etc.).
Now protons and electrons have charges. A charge is the sort of thing that gives you a shock when you pull off a clingy night-dress, and a charge is what makes your hair attract to a comb or a balloon stick to a ceiling. And just like magnets, charges come in positives and negatives, and opposites attract one another (+ & -) while like charges repel (- & - or + & +).
In an atom, the protons are positive and the electrons are negative, and it is this that keeps it all together and stable and generally from flying apart or collapsing in on itself, so generally there are roughly the same amount of protons as there are electrons.
Anti-matter is simply the same thing, only with reversed charges. So protons are now negative and electrons are positive. This means that it is perfectly possible to have an anti-matter universe that is entirely alike to our own, only the fundamental building blocks are reversed, (THIS is what they were going on about in Dr Who).
OK, so, an anti-matter hamster would be virtually indistinguishable from the real thing? Well…yes, almost. You see anti-matter in our own universe is horrifically unstable. It all comes back to the fact that there are different charges, and matter charges upset anti-matter charges so much that the whole anti-matter atom ends up falling apart. This is why anti-matter is so notoriously hard to make, it isn't just like rubbing a balloon on your jumper to make it stick. In fact it takes at least 10 nuclear physicists and a particle accelerator to make just one tiny part of one atom, and even then it only lasts for millionths of a second.
But these difficulties aside, even if you did make an anti-matter hamster, similar to types of fish that cannot be kept together because they kill one another, a matter hamster and an anti-matter hamster could not be kept together because they would neutralise one another completely (making breeding impossible). In fact, you couldn't even keep an anti-matter hamster in a matter cage, or even a matter atmosphere. You would have to either keep it floating in a vacuum (equipped of course with a little anti-matter breathing apparatus else it would obviously suffocate - though instead it will probably explode in the low pressure environment). Or perhaps you could keep it in an anti-matter cage suspended in a vacuum, however this is a little prohibitive to playing and cuddling your hamster, (the risk of exploding always being a bit of a turn-off) and even feeding would be tricky what with it requiring such complicated preparation and air-locking systems. However, the principle is there I suppose... and if you love your hamster enough, you won't care will you?