Why doesn't chocolate go bad?

Simple: it has no water.

Chocolate is a suspension of cocoa solids and sugar in cocoa butter. It is made from fat and carbohydrates only. Bacteria, as everything else, need water to live. They can't survive in something hygroscopic (like jam or honey) or something with no water at all (flour, chocolate, pure fat). Similar for molds. So, independent of temperature, chocolate won't go bad in the sense that it will never grow colonies of bacteria.

As Hobodave mentioned, it can "grow bad" in another sense. If held at high temperatures (somewhat above 30°C), the chocolate butter will separate from the mix and form a dull yellow-grayish crust on the chocolate surface. Also, if you leave it in contact with oxygen for long enough (many months), the cocoa fat will go rancid. In both cases, it is perfectly safe to eat the chocolate without risking food poisoning. However, the taste is much worse than in normal chocolate.

It does go bad.

The shelf life of opened dark or bittersweet chocolate is one year. Milk chocolate lasts only about eight months, due to the presence of milk.

The reason it has such a long shelf life, even opened, is due to the cocoa butter. Cocoa butter is a fat, but it is primarily a saturated fat, and thus is solid at room temperature. Saturated fats are naturally less susceptible to degradation than unsaturated fats (oils). Cocoa butter acts as a preservative in chocolate, as well as in cosmetics.

I'm not sure how big a factor it is compared to the other answers, but chocolate generally doesn't have water in it, and bacteria generally don't like water-free environments.

Most commercial chocolate does not have a shelf life. Batches are dated for processing and tracking purposes but there is really no inherent limit to how long it will remain shelf stable assuming it remains below about 30C / 85F and in a correctly controlled environment.

Chocolates that contain preservatives typically do so because they contain non-chocolate fillings.

Most chocolate that goes "bad" is the result of 3 issues:

1) It was in an environment above 30C / 85F will begin to lose temper (above 35C and 95F for sure) and begin to liquefy. Once the temperature returns below these temps, the chocolate will resolidify but may take on a chalky white texture and color. Assuming it didn't get hot enough to burn, the chocolate is fine and while it may lack correct mouthfeel, it is perfectly fine to eat. This chocolate, if melted down and correctly retempered, will return to its original, crisp and shiny texture and color.

2) It was stored for a period to time along with other foods or items that have a strong odor. Chocolate is high in fat and as such easily absorbs flavors and odors.

3) Storing chocolate in a cold location and then introducing it to "room temperature" will cause condensation on its surface. This moisture will later cause a "bloom" on the surface and depending on the amount of moisture introduced and over what period to time, may be purely cosmetic or have an actual affect on the chocolate but it is difficult to say at what point it has actually gone "bad".

Storing your chocolate in a refrigerator or freezer is a poor choice for reasons 2 and 3. Years ago I had a friend that stored chocolate in a motorhome in which he smoked and he wondered why after 6 months it tasted like an ashtray?

Both cocoa butter and the dark solids in chocolate are high in antioxidants. There is also almost no moisture in "tempered" chocolate. When chocolate is tempered the cocoa butter part of the chocolate is crystallized, which helps the chocolate form a tight lattice-type structure. The combination of 1) a high percentage of antioxidants, 2) almost no moisture, and 3) a dense, tight crystalline structure results in a product that has an extremely long shelf life. I make chocolate professionally, from the cacao bean, and I have some bars (70% cacao) that I make over 3 years ago that have shown absolutely no degradation. They taste the same as when I first made them.

Simple answer is that it does.

Most commercial grade chocolate (Cadburys and what have you) is full of various preservatives to keep it "fresh".

Also, higher quality chocolates will have less sugar in (as it will have more cocoa).

Its the same as bread - fresh, homemade bread will be spoiled in a few days, but supermarket bought stuff can last 7 days+

Chocolate must also have an anti-microbial factor because not only does pure, dark chocolate have an extremely long shelf-life but cakes made with high dark-chocolate content seem to last forever!

I've seen Trader Joe's Triple chocolate bundt cakes (NO perservatives) last over a month uncovered at warmish room temperature! Try that with any other natural cake or bread item... they get moldy in a week!

The only ingredient that they have different from other cakes is chocolate (and lots of it).

You'd have to define what "go bad" means exactly. If it gets toxic or whether the taste, looks or something else changes.

I found an answer from someone from lindt&sprüngli:

"Although cooling could probably prevent the chocolate to expire fast, room temperature is actually the best for the chocolate's quality. I still don't get why so many people like to put their chocolate into a fridge, because the taste of a chocolate is the best at room temperature. Anyway, basically a chocolate expires after six to twelve months. That only means, that the company which sold the chocolate can't guarantee that the characteristics mentioned above will still be as good as at the beginning when you bought the chocolate. That's it. But what if you wait even longer and let's say, eat a chocolate which expired ten years ago? Could you die? Most probably not!"

source: http://quandano.com/questions/can-chocolate-expire

Chocolate does go bad, when it does it's called blooming, the cocoa butter starts to run out of the chocolate and leaves what looks like white flowers or webbing on the surface of the chocolate.

Chocolate lasts longest when kept in the cold.